How long does it normally take to confirm 1 on blockchain?
You should get 1 confirmation within about 10 minutes.
Is it advisable to wait until that single confirmation has occurred before parting ways?
It's advisable to wait for at least 1 confirmation, but it's probably safe before. Have a read of this to get an understanding of double spending:
https://en.bitcoin.it/wiki/Double-spendingPersonally, I judge it by the value of the transaction, because nobody's gonna try and double spend for small amounts.
So, something like this:
If it's less than £10, it's probably safe at unconfirmed
If it's less than £100, it should be safe at 1 confirmation
If it's between £100 and £1000, maybe you should wait for 3 or 4 confirmations
If it's over £1000, wait for 5 or 6 confirmations.