There's so many factors to consider in this issue and you'll get a wide range of opinions and experiences, depending on where people are in the country, their DC's attitude towards the 11+ and whether they have tutors, DIY and have a combination of the two. Some people will say you should only do two practice papers, some say do some work for a year and definitely through the holidays.
Basically, it depends on when the test is, how many tests, how long you've been preparing and whether it's a superselective. DD2 has a place at a superselective: last year, everyone found out in Feb (or was it slightly earlier) that the test was being brought forward to September instead of November - that factor cut down our prep time. Coupled with a YR5 teacher who we discovered to our horror in May of that academic year, had focussed only on the middling and lower-performing sections of the class, ignoring the upper-level children, we had no choice but to work through the holiday.
Please come back and give us some more info and I'm sure you'll get some more advice.
For what it's worth - DD2 has survived the Summer Hols of 2012 and it's had no ill-effects
She's still a normal 11 year old who is lolling about on the sofabed, watching tv, eating Pringles and occasionally picking up the iPad