How long do you have to enter Thailand after approval of the DTV visa request?

Up to you once approved you have the visa no time limit on entry you could wait 4 years then go but you only have a year left then

You have 5 years from the date of issue - come, go - at any time in that period

I got mine and from the day of issue have effectively been using it to live (and work remotely) in Thailand for over a year now

But you could get it, wait months before you even use it once, hell, even longer if you want