Well dlybrand that depends upon which species of sphagnum you are growing. I grow it like this and it gets thick and nice.
You just need to keep it under a good light level.
Also it grows much quicker in pure water then on peat. And you dont have to wash the sphagnum off when you are going to use it for your nepenthes.
But as I have said on other topics about sphagnum then Glider you should some small amount in pure water and some on peat.