The best time to evolve Weepinbell depends on the moves one wants it to learn. Since evolving Weepinbell into Victreebel requires a Leaf Stone, you can wait to evolve it until Weepinbell has learned all desirable moves by leveling up, or at least until just before it would learn an important new move. For example, in some games like Pokémon Crystal, it's recommended to wait until it learns Razor Leaf at level 42 before evolving because once evolved, it may not learn certain moves by leveling up. In general:

• Evolve Bellsprout into Weepinbell at level 21.
• For Weepinbell to Victreebel, use a Leaf Stone when ready, ideally after learning key moves.
• Moves like Razor Leaf are good moves to learn before evolution for better performance.

So, evolve Weepinbell when it has learned the moves you want or just before it learns a big move, then use a Leaf Stone to evolve it into Victreebel.