As political scenarios go, the new Parliamentary position is just about as good as it gets for an Opposition. The Government is facing an almost impossible task (securing 'good' Brexit terms), is much too weak to push through much the Opposition doesn't really want, and has seriously lost its political momentum. Crucially, the historical precedents are heavily stacked against the Government surviving anything like its full Parliamentary term.
Let's put it this way. The current Tory Parliamentary position of being just a few seats short of a majority of 322 (that is with Sinn Fein votes subtracted, and they will never turn up) is almost identical to that of the Callaghan Government in March 1979. And that was the moment when they lost their famous vote of confidence! The Labour Government had become a minority one in 1977, having lost its majority of 3 (won in October 1974) in by-elections.
You will search in vain for a Government in the last century that has lasted for much more than 2 years without a single party majority. The 'best' example was the the 1929-1931 Labour Government that was sustained by the Liberals. But then that only seemed to last as long as it did because the then PM, Ramsay Macdonald was offering a real prospect of electoral reform to the Liberals. But the Government collapsed in 1931 in the midst of a national crisis. Remind of you of anything (that's coming)?
The Liberals survived longer after an inconclusive 1910 General election, but only because they had Irish Nationalist support to put through Home Rule legislation (which was ultimately short-circuited by the First World War). The few baubles of things like aviation taxes, infrastructure projects and an indemnity law for soldiers that the current Government can offer to the DUP do not constitute anything that compares anywhere near to that prize. Indeed, this time the complications of Northern Irish Government point in a negative direction! Once the political tit-bits are passed, the key incentive to support the Government disappears.
Although it is true that on technical grounds the Government can limp on so long as the all the DUP MPs back it in confidence votes until such time as the Government loses several by-elections (4 years?), the arithmetic looks too thin to imagine that Conservative backbenchers can withstand this sort of pummelling for quite that long. The DUP, who don't as a rule, much favour the Tories fiscal priorities anyway, are hardly likely to want to sacrifice themselves for the little that they will be able gain after the first couple of years at most. Because, short of a definite upswing in the UK economy, once the Government have put whatever mainly financial incentives in place for Norther Ireland, and Labour have indicated that they will not disturb them, the DUP will have no incentive to carry on supporting what may become a rather unpopular government.
Perhaps if the DUP agrees a pact to last a defined period, say 18 months as was the Lib-Lab pact of 1977-1978, we shall know when the next General Election will be (ie more or less directly after the end of the pact). A pact of more than 2 years covering 'supply and confidence' , even if it is signed on paper, may lack credibility and deliverability. I am writing this before we know how long the pact with the DUP will last, but I must say the announcement that there will be no Queen's speech after the imminent one until 2019 might be regarded as a giveaway. Another General Election in 2 years? That is, if they last that long......
Meanwhile the Labour Party can ambush the Government at times of their own choosing. They can draw in even DUP MPs to support them on many issues, and leave the Government with the ownership of an exit deal with the EU involving the UK paying a compensation bill of tens of billions of pounds. No doubt it will be a deal that satisfies nobody very much. Personally, I'd like a referendum on whether it is worth leaving the EU to have to pay that particular bill.
If a political party had to choose a time when it had to be in opposition, this is the perfect time for Labour!