I'm pretty sure that rarity plays the biggest part in that. My 4* and 5* tanks take way longer than the others to repair. Also, as you mentioned the level of each unit goes up so does the timer. Approximately a minute or so. Not sure about tank type, but that might have a small part to play as well. Heavy tanks take longer than medium tanks, and light tanks repair a bit faster than medium tanks. Destroyers and Artillery..I think they are around medium tank range in repair times, but not 100% sure.
Does anyone know of a good, resource efficient, grinding spot? I'm kinda stuck on level 20 areas right now.