I would say as quickly as possible because the only good move it seems to have is Thunderbolt at Lv. 26.
After level 26, where it learns Thunderbolt.
None of the later moves are of much use:
29 Feint (there are better moves)
34 Agility (Pikachu/ Raichu's already fast and too frail to set up)
37 Discharge (you already have T-bolt)
42 Light Screen (possibly useful, but most ground moves are physical anyway)
45 Thunder (low accuracy, stick to T-bolt)
Pikachu Gen IV learnset