When I did primary instruction full time this is how I did it..
Usualy around 15 to 20 hours the student has demonstrated HIS/HER ability to fix airspeed, pending stalls, botched landings, with out my help...that's key...so when it came time to consider the person to solo..I did 3 supervised solos..
The first one usualy comprised of my flying with the student for about an hour or so...turns, climbs, descents, holding airspeeds and alts, bascialy do this and that, and the student does, and fixes his own problems, then slow flight then stall recoveries...if all goes well, everyone feeling good, we do like ten landings...finding a runway with no wind, plenty long, no traffic, and I sit back and have the student do his thing with very little imput, seeing if the student can handle it on his own...ten landings later the student, should feel pretty good that all he has to do is replicate three landings on his own, having just done ten landings of the same...
I get out, I watch, and the student does two touch and goes, the last one a full stop..he picks me up...now he feels like a pilot..he flies me home...
As far as when to get out of the plane for the first time...sometimes an instructor has to guage the confidence level of the student...as they have already demonstrated flying/landing the plane so much with you, this is simply about doing it themselves...so you need to give them the chance to do that when you fly with them...don't be grabbby and always talk them through it..too many instructors barely ahead of thier students, don't have the confidence to let thier students actualy fly....
2000 hours of instruction I only had one student almost blow it...simply put, 22-30-40 hours later, he was very proficient, but lacked any confidence and almost busted up the plane...it was all psychological..once I was out of the plane, he just didn't think anything he did was him....my mistake was doing one of those 'ok it's like 40 hours, time for me to get out'...kind of like throwing your kid in the lake when all else failed to teach him how to swim...the fact is...that rarely works...if you have done everything you can to demonstrate they have confidence in the plane, and they just can't do it...then your in the area of how they were brought up..being thier psychologist basicaly, and not thier flight instructor...all that said, I had plenty of students other's gave up on, especialy older peeps wanting to fly...it's fun and challenging finding ways to get people over learning plateus, psychological demons, etc...
Anyway, you do three of these supervised solos, picking days where you have a little more wind, some more traffic in the pattern, where they demonstrate fixing thier own problems, building confidence....and if they they show themselves feeling good, being solid, no issues, then you restrict them to come out on thier own on nice days, little wind and work up from there...
In the mean time your still flying with them a few times a week, and they are giving you feedback on thier solo flights...critiques, thoughts, feelings ect...
Most of my students were done by about 60 hours...