Is there a timer on available Garrison missions?
Solution 1:
Yes they do.
But there doesn't seem to be a general rule of when missions expire, I personally experienced that some missions expired after 2 days, but people over the internet are arguing about how long it takes to expire. After checking in-game I also did some research but there's actually no expire date displayed in-game. This makes some people think that the expiring of missions might be a bug, but there's no official statement from Blizzard about that.