About 4 events per month. If you can get 3x 3-stars per week then that is 12 you gain in that month and that does not include SC gained from story missions, daily missions, revival events, and login bonuses. By the time they outgrow those buffs, they will be able to consistently gain 4 SC from each event until they can clear the hardest mission. Those buffs can also help them clear story mission sooner, allowing them access to more efficient maps they can auto-complete. You also don't need to buy the full set of buffs. You can get through most missions with the damage reduction buff unless you need more damage.

After a month or two, they should have become strong enough to clear most missions without the buffs unless they need it for a Star Trial with a difficult mission and they want a flawless event unit. It will still take many months until they can clear the hardest missions consistently, but that is just the nature of these games. On the bright side, they were able to farm for those fabled event units they want so badly.