Difficult to give an answer to the number of hours before entering Standard level competitions. But DB6s answer is about right.
However, I would add that there are beginners competitions and L**p days run by the BAeA. see
http://www.aerobatics.org.uk/sites08...02008%20v3.pdf
These are aimed at pilots who have completed the AOPA course. Most people do one or two of these and then go into Standard level.
ZA