I think the first 4-5 rolls can be made pretty quickly (20-25 SC) before you decide to reroll an account.
Here is an easy estimation, 5 rolls in if you don't see a plat, you can consider a reroll. (5x10% - 50% chance of getting a plat)

By your 2nd account, you should decide which one to keep. No point constantly rerolling, there is no progression. Dmm gives you 3 plats and other goodies anyway. (additional plat sailor which i don't use because dmm gives you more units than nutaku..getting 2 free blacks from events)