... Huh? If I read it right, you mean when you should catch and train another Pokemon? If that's it, get the next elemental. Fire/water/grass. Then get a flying or ground. Then dark or bug (E4 always have one or more weaknesses to bug or dark or fighting [get that too]). Then train them up.
The duration you will have to wait before getting a second Pokemon on Pokemon Black 2 is not given. However, when you reach a certain level, you can be easily allowed to get the second and the third Pokemon.