First of all, measure the chain with a 12 inch ruler. You can safely run the chain until, it's stretched 1/8" over 12" (1%). Then you have a decision. SS bikes are very immune to the skipping that plagues derailleur bikes, so odds are the chain can run to about 3% stretch (3/8" over 12")and still be OK. That might take a lifetime, or if sprockets cost more than 2-3 chains, you might use a new chain at 1% or so.
Replacing the chain sooner than 1% would be wasteful, unless chains were very cheap.
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