Good question I think it gives you a chest based on how many you got but the exact number I do not know!?
I believe I found the answer elsewhere and they don't get saved like stars -- it's use them or lose them within the context of a single trip to infinity. I hope someone can confirm whether or not my understanding is correct.
The time shards mechanic is rather simple, you collect them -> you get a chest based on the number collected -> you start another run trying to do better.
Or in other words you automatically buy the respective chest at the end of your run.
Other than that the chests don't get better once you reach 25k shards as far as i know, so if you are only after the legendary stickers all you need to do is get 25k shards in a couple of runs.
