Meh, you can get 60 summon scroll a month fairly easily, then add the rebate event in to go along with the daily free draw, I'd put it more like 18 month to 2 years.

Granted, your income will increase overtime. For example, once you are done with medium refine and thread on monthly, seal and summon scroll are kinda the best things (unless the ninja frag offered is one you want, it's pretty bad ratio in there compared to other event/source but it's an option) so you'd end up with more per month.

So... I'd say 15 to 18 month most likely.