Some where every 800 - 1200 usually. You seem to have gotten rather unlucky, so I would wager that you'll probably get your 2nd on the lower end of that. My first was 1170, but my 2nd was 1080.

But it is possible you might get really unlucky and not get it till 2500. Just make sure you take advantage of doing 300 pulls. Those extra 66 seal make a big difference in the long run.

is 500 worth to get 2 hashi frags

yes it is, if your goal's is Hashi, it's like 100 SS free pulls if we assumed every 50 pulls you get 1-2Hashi frags.

No. 2 Fragments is not worth 200 seal scrolls. Save it toward the next 300 rebate.

Technically it could be- if you're purchasing your scrolls, then yes- You get a Hashirama fragment every 30-50 seals that's accurate as I could get it.
If you're saving your seal scrolls, then over a long period of time, the 200 saved toward a 300 rebate is going to be more beneficial in gaining your SR, we're talking about roughly 2500 on the low end, to 3000 & upwards to complete Hashirama, unless you're spending that's nearly 2.5 years away it's not worth it unless you're spending, at least that way you're gaining those Extra Hashirama fragments..