Most Pokemon evolve after they hit 220 happiness.
There's not a certain level a Pokemon evolves by happiness by, but if you just want a crobat, I think I can help.
Here's a list of things to make your Pokemon happier(In B/W):
1:Catching your Pokemon in a luxury ball: 1.5 X happiness level.
2:Giving your Pokemon a sothe bell: 1.5 X the happiness level.
3:Walking 256 steps: +1/unknown.For 0-99,100-199,and 200-max level.
4:Massage person in Castelina city:+3 for 0-99,100-199,and 200-max level.
5.Using stat vitamins:0-99:+5,100-199:+3,and 200-max:+2.
6:Leveling up your Pokemon:0-99:+5 or +6,100-199:+3 or +4,200-max:+2 or +3.
7:Useing an EV berry:0-99:+10,100-199:+5,and 200-max:+2.
1:Fainting:-1 FOR ALL HAPPINESS LEVELS.
2:Giving your Pokemon energy powder:0-199:-5 and 200-max:10.
3:Giving your Pokemon heal powder: same as giving your Pokemon energy powder.
4:Giving your Pokemon Energy root:0-199:-10 and 200-max:-15.
5:Giving your Pokemon revival herb:0-199:-15 and 200-max:-20
Trading your Pokemon resets it's happiness level to 0.
My tip: Catch your Pokemon in a luxury ball and get a sothe bell from the lady in Nimbasa.