That depends on the type of Pokemon that is leveling up.
a Pokemon that has to be traded to evolve can be any lvl you want it to be you get it the Pokemon the same lvl that your Pokemon was it does not grow a lvl when it is traded
reshiram does not evolve
lvl 36
lvl 20
It evolves at lvl 45.
lvl.18 or lvl.16
at lvl 30
idalia
lvl. 30
lvl. 15.
tallows evolve at lvl 25
Pokemon evolve differently every time.