Family, Other when can a petitioned spouse enter the US?

[nyvisitor]

I understand that when a US citizen petitions for a spouse overseas, there will be a visa petition number immediately available if the petition is approved by the USCIS. Is there a quota for spouse visas regardless of country of citizenship? I've been told that there are visa "oversubscriptions" from China, India, etc. and that it takes years to process the visas for immediate family members. If a number is not available, and the spouse being petitioned is from China, does it mean husband and wife cannot stay together in the US until that number is given? When can a petitioned spouse enter and work in the US?