Usually, people recommend saving them for Summon Rush during a fusion event, especially for newer players who don't have the luxury of having a lot of better shards to use.

Do you know when we get the next Summon Rush with a fusion event?

Every fusion / fragment event has one Summon Rush and one Champ Chase event. Or is your question when the next event will be? In which case, the answer is, we don't really know - they seem to have indicated there'll be a bit of a break before the next one, so it's anyone's guess. Personally, I'm thinking mid-May.

Thanks.  

Kram, where did u see a break til mid may? That would be best news ever, lol. 

No break that I know of. Was GOING to be one this past month, but we got Nari instead.

I could swear I saw somewhere that after Nari they were going to pause for a bit? I'll have to dig around and see if I can find where ...

Could have been me, here (eep), since I was told before Nari that they would be chilling out on fusions for a bit. Or a CC since they were told the same thing.

I don't think they like pausing, as giving players time to stockpile resources means they'll spend less during the fusion itself.