+1 on Zib's comment. If you only have one strop, dont paste it at all!

If you have a second pasted strop then apply the paste once and then, as Sham says, reapply it when you feel that its cutting power has been reduced through use.

When I started I was using a pasted strop as a touch up method and so far I've not had to reapply the paste at all. The original paste is still going strong.