The above Training Pyramid graphic shows how you should structure your individual sessions when using a zonal training system. You can check out my previous blogs written on How to Train Using Zones, which will also give you some background on the system I use (sadly there seem to be many!).
The basis of this is that the higher the intensity of the session the shorter the duration. Your base endurance rides should be lasting at least a couple of hours, with sustainable power (but still aerobic (with oxygen) endurance) should be between 1-2 hours and the zones above that (where you become anaerobic (without oxygen)) even shorter.
Not only should the duration of the session get shorter but if you are using an interval system the intense interval will have to be shorter too. I wouldn’t recommend using intervals for zones 1 or 2 just go for a ride to train in these. Zones 3 & 4 if using intervals you can go from around 5 minutes with the intension of being able to ride for between 1-2 hours steady depending on what part of the zones you are in (although this can be tough & fatiguing & might take time to recover from). Zone 5 intervals can be kept shorter still so you can go from 1-8 minutes taking a similar amount of rest or recovery in between each interval. Zone 6 will be similar to 5 but intervals might last no more than 5 minutes. Anything in the supra-maximal zone (above zone 6) will be very short sprint intervals.
You should always consider warming-up to the frequency of the session, a zone 1-2 ride might not require one (or do it by progressing cadence on the bike) while a zone 6 session will require a thorough 20 minutes warm-up. Cooling down is important too so you kick start the recovery process and are as close to 100% for your next session.