This is a bit of a complicated question.
Assuming that in one year, there will be 10 people in each house, which is roughly the number we get from the books, there would be about 70 students in each house. In total, Hogwarts would have 280 students.
In interviews, though, JKR said there were about a 1000 students at Hogwarts. This was later revised it down to 600, which - assuming a roughly equal division - would mean each house has about 150 students.
Quite a difference there - but then, maths isn't JKR's strongest suit ;)
The Harry Potter Lexicon gives a more detailed analysis.