Actually, if you're only planning on leveling two teams to 60th...
I cannot see that taking more then a month.

When you pay for a month with the referred account, it credits the referring account with a month.
If you're done at that point, you lose the Zhevra mount because no 2nd month has been paid for.
But on the other hand, you've bought 4 wow's and essentially paid for 4 one-month subscription extensions on existing accounts.
Not much of a cost involved, for 1/3rd off of leveling time (2/3rds time per team).