Never, a properly glued sew up never needs to be re-glued for the life of the tire, which is iether a puncture or the tread wearing out.
I guess if you hadn't ridden a set of wheels in 5 years, the glue might have gotten so brittle that it needs to be reglued, but in normal use, you will flat or wear out the tire way before the glue gives up.
If you have any doubts, just test to make sure the tire is properly glued. Inflate the tire until it is just firm, grab the tire and rim with both hands and try as hard as you can to roll the tire off the rim. If you can get it to start to peal off the rim, then re-glue it, otherwise, you're ok.
Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1