It usually posts within about a week, if I recall correctly.

Note however, that you'll get better value if you transfer the topcashback money to Tesco Clubcard and then covert from Clubcard to Avios. However this will likely take much longer, since Tesco only issue Clubcard coupons each quarter.

With Clubcard, I think you get 600 Avios per £2.50. Doing direct from topcashback to Avios you'd get only 250 Avios per £2.50.