The general advice is 4-6 weeks, but that's an awful, awful long time....
Personally, I've never had a limit. When a cat appears settled in the house (usual minimum a week), I do the following...
Wait until the cat is really, really hungry. Go outside first - before you allow him out. Then gently open the door and let him follow you. All the cats I've ever had have been a bit cautious when first allowed their freedom, so I use that. Stay with the cat, follow a few feet behind - that's really important! If he looks like going near a fence - scoop him up. After a few minutes, they've usually had enough and start thinking of their tummy, so I pick them up and take them back inside. I do that at least 5-6 times before I let them out of their own. Start with just a few minutes, then build up.
I'm not convinced about the harness idea. Mine do a runner when I come near them with a harness, and you will, eventually, have to let Max out without one.
I think it's a case of you having to know your cat well. You'll know if he's ready. The blanket suggestion of keeping a cat in for weeks and weeks in impracticle in my experience - the more you try to imprison a cat the more it tries to escape and that isn't going to change with time, only get worse.... All they're doing is getting used to their bearings so they can find their way back - your door looks very different when viewed from the garden. So do bushes, trees... Like with everything else, overload them with knowledge and they'll forget it all. But once puss knows full well where his bowl is, or where the tray is, or where you keep your vacuum cleaner, he's ready to learn more....
Best of luck
Juliexxx