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