I have made a set of studded tyres.They work very well.How low should i let the studs wear down before i replace it?

Until they stop working.

Just retired a tyre after 15,000 plus km and 10 years as the casing and tread were actually degrading while the studs were still fine.

Ok,I`ll Keep riding till it gets a bit slippy,thanks.

The tyres I am running tonight have 5000km on them and still bite like a mad dog...

You could test it on some black ice.
Ive been thinking about wear rate and how easy it would be to replace all the screws. I dont think it is a major job, about 2 hrs.
Each screw head is backed by a cut out square of inner tube and these would have to be removed and reglued (using Copydex rubber adhesive). I tried gluing a whole inner tube in place but it is easier with smaller bits.

I have a second tyre with the bead cut off as a liner.Easier for re studding .;-)