database must be vacuumed with <N> transactions

http://www.postgresql.org/docs/8.4/static/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND

The database I work on started spitting out the following message last
night:
WARNING: database "ops" must be vacuumed within 9361597 transaction

I ran VACUUM as soon as I got in, but then shortly after, it stopped
accepting transactions. I stopped the database and opened single user mode
to run VACUUM. So my questions are:
1) How long should I expect this to take (database has about 5 TB of data)?
2) Is there a faster way to make the database be operational again?

Thanks,
Dave