Never heard of Clearit, but it does have the same ingredients as the Thornit I use, so I imagine it is the same stuff
A bottle of the powder lasts for
ages I have a 20g bottle which is more than half full and I have written on the label that I got it in November 2014
I am a bit slapdash about using it regularly, but you don't need much.
It looks like 20g is the only size they do, but if you can get less, then I would get that. Either way, it will last you ages.