How long do you need to keep 500,000 baht in your account for a Thai DTV visa application?

up to 6 month, depends on consulate

It needs to have been in there for three months minimum prior to your application. That's the hard rule but with this visa being as great as it is ide say try and hold onto the cash in case your ever asked again for it in future or something changes. It's the main thing they want from this visa. You should be fine otherwise however I think if you can't. No stress in sure others have spent money since also. What their trying to stop is the decades old grey areas abused by non visa holders, IE the border runs just to get another 60 day stamp, with another extension then border run again and so on. If your on the correct visa you should have zero issues for the rest of that visa. I tend to go overkill on the evidence stuff just in case but it's not been needed. Didn't even get asked for the 20k baht cash evidence on my way back in after doing a same day airplane return to Malaysia but still withdrew it before leaving just in case.

No I literally put it in the 31st so it would show on the new statement 24hrs later

Generally, you'll need to have the funds in your account for three months to meet the 'seasoning' requirement for a DTV application. However some foreign missions may not have this requirement or may require six months. It's best to check with the specific embassy or consulate for their rules.

No seasoning is required after the visa is issued.

Not in theory. Said so, why would you need to deplete the account?