when you can, for longer if you want him to take a longer nap. by carrying him just half an hour longer (so 1 hour in total) you will have helped him through to the second sleep cycle. if you put him down then, there's a good chance he can keep sleeping for another 30-40 minutes.
ps: have a look at my "nap in a snap" guide. it has easy tools to set up the best-fit nap routine for your baby ... and to help him sleep those naps well too. 