Time to replace the rubber

Hi Alois

I thought maybe someone else had asked this question before but couldn't find it using question search....apologies if this is an old question.

I was wondering when it is time to replace an older rubber.  Is it basically when the top sheet wears out?  Can sponges wear out as well?  How do you decide when a top sheet has passed its useful life?  When it is so worn you can see the sponge through it?  When it loses all friction? (i.e. you can slide a ball across its surface as though it were antispin?)

Thanks!

Ji-Soo