Every tech related forum should have it IMO. 180 days seems like a good bet for this particular forum since topics don't tend to zombify so quickly. For a Stackoverflow or an OTN I'd have opted to set the deadline to 2 months if it were up to me.
Include game threads? (WIP, showcase, featured)
For the WIP its actually a good thing, keep the progress going or have your thread necro'd
The other two are good points however, you'd want people to be able to leave a congratulatory message or ask a question no matter how old your game gets.