As you should know by now there will be 7 HP books in total. Number 5 is about to be released. There's been a 2 year gap between 4 and 5, so presumably we can expect about the same between 5 and 6 (coming out in 2005) and then 6 and 7 (2007).
Now, film 3 is out in June 2004 (a whole year away!) and presuming it takes 18 months to make a film we can expect film 4 to be out at Christmas 2005.
Except that it's been mooted that film 4 will be 2 films (not entirely surprising when book 4 was 800-odd pages long), thus making even more money for Warner. Goblet of Fire would come out Thanksgiving/Christmas 2005. Order of the Phoenix would be June 2007, book 6 would be Christmas 2008 and the last film would be 2010.
This would keep pace (just about) with the books (barring some disaster) and it would mean that the actors would be around 21 when the last film came out, a difference of only three years with their characters (just about feasible with the magic of makeup).
Darnit, whoever heard of waiting 10 years to see a whole series of films complete?