servant leveling

Maybe around 30 to 40. Theyre not very effective at lvling and they cost more qp.

I only use gold cards and sell the 3 star for mana prisms

Some three star servants are still worth for LV 70.
Some servants are not require to maxed level because of their growth curve. Like Cu alter's lv60 status is only 100 hp and 200 attack fewer than lv 70.
However, materials is somehow not profitable for leveling their skill up to lv10. Only only few like Euryale, dual Cu, Medea, David have skills are worth to lv10. Save all the thing for SSR and SR.
If you posses some servant like Gilles and Geronimo, then you don even need to level them to lv2 if you are in short of QB, materials and EXP.