Bigpikle, How many you can do really depends on your current fitness and whether your FTP and consequently your "levels" are set correctly. Your body will let you know when you're ready for a rest - either sore legs and a need for more shut eye, or more subtle things like loss of appetite, lack of desire to go riding hard etc. JS, There's many a good workout that can be had in 90 minutes - whether it be a full on FTP effort for one hour with warm up/cool down to a couple of 20 minute efforts combined with 5 minute-ish VO2 max work and some 30 second springs done at full gas. While I agree with you that you can't really cheat your way past training, you can plan a really good training block with sessions of 90 minutes max. The key is keeping to the sessions and not adjusting them ad-hoc. 
If sessions start to feel easier then its time to stick to the planned session duration but either re-test to check for higher threshold and adjust accordingly or just bump the desired target up 10 watts and see how it goes. At some point you should really re-test. At the end if the day it all comes down to commitment, talent and desire. Most of us on here will never earn most of our money riding bikes and many will regularly never ride more than 100 miles. You certainly don't need a large block of 2 to 3 hour rides to do the odd 100 mile ride - especially if you have an FTP that's been enhanced by some really hard aerobic work. Sure, it might be mentally easier to do 3 hours mix of L2/L3 out on the road than smashing high end L3/lower L4 but both will get the job done for the average club guy. In some ways I feel that doing too much L2 work for the guy on limited time might not be the way to go unless there's a dedicated block of training for L4 later on. As simple as it sounds, if you want to go fast, at some point you really do have to go fast. Sometimes I think that this point is missed.
If sessions start to feel easier then its time to stick to the planned session duration but either re-test to check for higher threshold and adjust accordingly or just bump the desired target up 10 watts and see how it goes. At some point you should really re-test. At the end if the day it all comes down to commitment, talent and desire. Most of us on here will never earn most of our money riding bikes and many will regularly never ride more than 100 miles. You certainly don't need a large block of 2 to 3 hour rides to do the odd 100 mile ride - especially if you have an FTP that's been enhanced by some really hard aerobic work. Sure, it might be mentally easier to do 3 hours mix of L2/L3 out on the road than smashing high end L3/lower L4 but both will get the job done for the average club guy. In some ways I feel that doing too much L2 work for the guy on limited time might not be the way to go unless there's a dedicated block of training for L4 later on. As simple as it sounds, if you want to go fast, at some point you really do have to go fast. Sometimes I think that this point is missed.
