How much stardust should a level 30+ player always keep on hand?

you prob should keep arround 500k i guess, however i'm lvl 35 and like to spend on mons i like.Spending my dust give me the motivation to keep grinding. play the way you like the most :)

I'm 35 and I just emptied my 500K dust on Pokemon to get higher spots in gyms. Next on my list are Snorlax, Lapras, Espeon, Donphan, shiny gyarados and Machamp.

I don't see the need to hold any star dust as back up. I just spend it when I feel the moment is there.

I like to keep around 300k...although lately I have dropped quite a bit as I am powering up a new Tyranitar and a couple of Machamp, and my Blissey army ate a lot of dust after I leveled up during the dbl xp event.
For me it is time to collect and save up a small hoard again so that I can be ready for the expected gym shakeup, because I don't like not having a reserve.

1. There is no need to store any specific amount of stardust.
2. There is no need to spend stardust just to spend it.

Use it as you need it.

It really depends on your local gym meta and how happy you are with your current attackers/defenders.

My gym scene is very active and highly competitive. I have 150K stardust and see no reason to hoard when I have a Machamp with Count/DP to power up (as I earn buddy candy). When I finally get a wonder Poliwrath with RS/DP, I will blow stardust on that too.

lv 33 5240 stardust

As Simon stated there is probably no need to keep any amount or to spend just for spending, my advice is spend it wisely only if you have to.

In my case I am at 8 stable gyms since two months ago, no need more defenders or attackers since I can get 80-100 coins per day, I am almost level 37 and have been saving since I knew about the incoming gym rework, I prefer it this way because I have candies for 2 more Dragonite, for a Tyranitar and for an Ampharos and I want to have dust to power them up to the max as soon as gym reworks hits and I know what are the best movesets at that point and how the new system works, just for this reasons I have now saved 1.2 milion dust, but just because I know there is that gym balance around the corner.

Before this I never had saved more than 200k-300k, that was the amount needed to get one mon to the max of my level, of course that amount was saved for example the case of a hatched 100%IV Dratini, or a 90%IV+ Snorlax, or a 2 ice move Lapras (still don't have one with double ice moveset after 9+ months of playing everyday...)

I use it as soon as I get it; I have the mons I want to invest in

I keep nothing. I live for that next power up.

As much as you have left after powering up the pokemon you want to (to level 30). It really is a matter of preference and what you have to power up, obviously if you have multiple high IV good moveset Dragonite, Blissey, Snorlax and Tyranitar, you are less likely to save dust. There's really no correct answer here.

I reached level 35 during the eggstravaganza event.
Don't usually save any stardust, but I guess that having always handy 200-300k would allow you to power up/max out 2 extraordinary hatches almost immediately.
I'd assume that a level 35 player already has a good defence and attack teams, nearly maxed out. So, he/she could spare stardust on favourite mons and keep always defenders well nourished to get the best spots in gyms. At least that's what I'm doing.

I try to keep whatever I earn in an average three week period so I can proactively react to changes or great hatches. I'm kind of in between power ups right now and have about 800K.

I spend it as needed.
I spend it to keep my defenders high enough to hold gym spots.
I'm more a prestige player than I am an attacker. The result is I really don't want to power up attackers, I need them to stay low, in order to prestige.
As a result, I have lots of stardust in reserve. Almost 800k. I catch around 200 mons a day, so I can built it up pretty quickly.

Up to level 31, I spent all my stardust to meet my power-up needs.

I am now level 33 and with the increased time between levelling up, I have had the "luxury" of saving up to 270 k of stardust. However, I use this when I get a new mon to power up and it can go quickly - I am now down to 140 k.

I think that I would now feel uncomfortable with less than 100 k as a buffer.

I have 1million, lvl 32

I am level 37, and I have 980,000. Hopefully I'll go over 1 million for the 2nd time.

I don't feel like I'm really saving it, but I catch everything in sight so maybe that's why I have so much. I am paranoid about Niantic changing things up on us, so that I will suddenly want to power up a bunch of Mons.

I feel very poor when I just spent a ton of dust. Even though the dust means better mons . So I like to keep at least 50k reserve for emotional reasons .

I base it like this:
If you are level 31, your defenders at max are level 32.5. When you make level 32, you can level them all up twice to get them to level 33.5. That will cost you 13K dust each. (6K from 32.5 to 33, then 7K to 33.5)
I have 12 defenders that I am trying to keep at max. For me, it cost 16K each when I hit level 35. 192K dust right there. You can use the table here:
https://pokemongo.gamepress.gg/power-up-costs

This doesn't count any new acquisitions (Like a Larvitar) or attackers that I might want to give a boost. Then, there's the treat of the future gym changes messing up my menagerie. All this taken into account, I am holding 750K in dust right now.