2019 FreetoPlay Hell

If you love them, roll, is the best advice. Gameplay-wise, seeing as you don't have a 5-Star assassin at the moment, King Hassan will be nice and compatible with your existing support lineup (Waver and any Merlins you borrow), as well as being a top-tier contender for best single-target assassin.

That said, I advise you to see the summer servants (including the ones for previous summer event) and see if any of them pique your interest more, particularly the 4-stars like Summer Nitocris or Summer Nobu, since they're more likely to come home when you roll. If they will fill the holes in your lineup or voids in your soul more than King Hassan, then don't roll until then.

King Hassan will come back eventually, though both times he returns he'll be the only thing you want on the banner, meaning the chance of being disappointed by what you do get will be higher overall. If you intend to remain F2P, that can also mean trying to minimize the chance of an unwanted result (i.e. a trash roll) with the precious quartz you have.

He is useful to you since it looks like you only have Scathach for an assassin. Eresh comes in December, and summer Fran comes in July/August (assuming we catch up a bit to JP the same way we did last year). King Hassan has one banner later this month and another in November. Imo his second one is a bit too close to Eresh for comfort, so if you are going to roll for him , roll for him this month.

If you are willing to take a risk: roll for King Hassan when he comes out later this month, then save until Summer banner 1. After that, save again until Christmas.

By my (rough) calculations, you should have something like 327 free SQ and 110 tickets in time for Summer, including the ones you get from the Summer event itself.

From Summer until Christmas you get 227 SQ and 52 tickets. These calculations have not factored in cumulative total login bonuses (where you get 20 SQ every 50 logins), master missions (these change a bit this year to give more quartz), and servant interludes/rank-up quests.

Assuming you don't roll for anything else this year, that should give you a pretty good chance at everything. Also, I doubt it'd take that many rolls to get summer Fran, so you should have a decent stockpile of SQ when KH shows up for the second time. I'd say, if you didn't get him the first time, roll for him the second, but leave around 300-400 SQ for Eresh.