We tell adopters 3 weeks but I know most give in after 2 weeks.
I myself, along with lots of other people I know who are very experienced, never make it very long! The howling and attacking the catflap as well as the knock on affect of limiting the other's access makes it difficult! I always judge it on the cat- if confident and bonded with us quite quickly then it's quicker to go outside and if more timid, I keep them in longer. As gillian said, them deing desperate to go outside really doesn't do anything for the bonding process and can really stress them out.
I do think 3 weeks is a good guide as it's long enough to get them aclimatised as well as not being too long without outside access. Ours vaired- we had Oscar was a kitten so was in for a long time, Monty was a fostered cat so was with us for 7 weeks before he officially became a housemate and went outside. Jasper broke his way out of the catflap in just under a week and by the time we realised he had worked out how to unlock it, he had been coming and going for a couple of days (such a clever boy!) we were planning on 3 weeks though. Our newest boy Marley went out after 10 days as we knew he was bonded with us and he was getting so distressed at not going outside. Again, we were planning on 3 weeks but judged it on him.
If I only had one though and it didn't affect the others then I would be keen to keep newbie in for the full three weeks/ four weeks.
Enjoy him!