How many seconds of nonstop voice (or silence) without taking another breath is satisfactory for a professional singer ? Is there a rule thumb ? does it depend on the register ?
You should be at least able to delay your breath in such way that it does not interrupt with your singing. Being able to sing more is a big bonus, but we can't really put a value on that: Just as long as you can...
You shouldn't be trying to sing more than you are able to, as that will also interrupt with your singing.
Lower registers need more breath pressure, so they need more breath to do the same.
But, unless you were to compare yourself with someone else, registers shouldn't really matter...