The average at our field is "most of the summer" to solo. By that I mean start coming out every weekend that its not raining or too windy in June and by about the middle of August or September most have passed their solo test. Fuel wise it works out to about 2 1/2 gallons. The reasons that a few don't finish to solo are: too unreliable an engine(Super Tigers act finicky at our field and the carb barrel jams). OS or Thunder Tiger are are a good choice. Trying to use planes that aren't trainers or are too advanced such as an Avistar. Some can learn on them, others find the plane too frisky. Over powering a trainer (mounting an OS 46FX on an LT25, makes the plane keep cruising along even at idle and won't allow the plane to lose altitude to land). Using small underpowered electric trainers. Can only be flown in very light winds. Is small making it harder to see what the plane is doing and has shorter flights than glow fuel. Also difficult to get the smaller planes up to takeoff speed on a grass runway. Requires either a recharge or battery replacement at the end of each flight. Hope this helps.