Takes way less than 28 months. consider you do rebates every month for 80-100 scrolls tier after a certain point. You hit 2k, very likely, around your 16-18th month of playing.

The question for you is: do you actually need that summon because without it your op team does not work? Then do not wait and buy it, but if you do not need it (are not actually many the teams that strictly require koh or kirin to work fine) then you may just wait for the first super rare to come and buy the other one with points while, in the meanwhile, you hoard 4-5k summon cultivation runes. If you do so you lose zero power.