I would guess 12 years, but a lot depends on many factors. For starters, although a Tory majority seems very likely, if the Liberal Democrats perform well in the election, we could see a minority.
A lot depends on how the Tories do. If they do a good job it could be longer, but if they do a lousy job it could be sooner.
If the centre-left (Labour Party and Liberal Democrats merge, thats assuming their results are very close) it could be sooner as well as if David Cameron turns hard to the right that could create a polarized electorate since although around 40% of Brits would like to return to the Thatcher Era, there are many who despise her policies, thus creating a polarized environment.
By contrast, the Labour Party could fall into disarray as there is a strong divide between the centrist elements (New Labour) and the more left wing elements (old Labour). Up until now Old Labour has gone along with the more centrist policies since they have resulted in winning elections, but that could change once they lose. The centrist elements will argue that it is the only way to win, while the left will argue the party needs to return to its leftist roots. Off course, Old Labour policies will only ensure it loses as Britain is far less unionized than 30 years ago and the hard left is considerably weaker than it once was, so their policies would not have the appeal they did in the 60s and 70s.
Dont assume that all of the "Old Labour" wing are Left Wing. People like Roy Hattersley are "Old Labour" though he is hardly Left Wing.
