Assuming you mean the British one, the only rule is that the maximum length of a Parliament is 5 years. There never HAS to be a general election unless 5 years have passed since the last one.
Before 2011, the Prime Minister could ask for a general election at any time and the Queen would say yes. In the case of a minority government, no doubt the Prime Minister would do this as soon as it became clear he can't get anything done, in the hope of winning more seats. I remember the last time - we had 2 general elections in 1974 because at the February one, nobody won a majority and no coalition could be formed. So Harold Wilson tried again in October. (LOL I was only 9 but I always remembered elections - my primary school was a polling station so elections meant a day off!)
What happened after that is instructive. Labour won an overall majority of 3, soon lost it in by-elections and kept going through the "Lib-Lab pact". Eventually the Liberals withdrew, everybody was expecting Jim Callaghan to call a general election in the autumn of 1978, but he went on TV and said he wasn't going to. That government finally ended when, in early 1979, the combined Opposition went for the nuclear option - a confidence motion which they fully expected to win the vote on, and did, by one vote. BONG, instant general election, the Conservatives won by a landslide and hello to 11 years of Maggie Thatcher. (She got lucky - Labour spent most of the next decade ritually disembowelling itself and going raving loony communist, so despite her unpopularity, every time she went for an election, the country looked at Labour, thought "oh sh**" and went for Mrs T again.)
So there's your illustration of the answer as it used to be. That government ran on a knife-edge the whole time and nearly managed the whole 5 years. No actual necessity for calling an election.
Unfortunately, when the Conservative-Lib Dem coalition was formed in 2010, one of the prices the Lib Dems insisted on to join in was the Fixed-term Parliaments Act 2011. This removed the power of the Queen to call elections except if the House of Commons votes for one by a 2/3 majority or the government loses a vote of confidence.
I thought it was a stupid idea at the time and now the chickens have come home to roost. It is now impossible for the Prime Minister to just ask for an election as before, he's got to get 2/3 majority for it, and the Opposition can use this to totally screw things up.
This can be the only reason, whatever Jeremy Corbyn says, for refusing to vote for a general election when Boris proposed the motion, given that he's been going on for months that we should have one, Now it's been offered, he won't support it. He just wants to make a mess and try to blame it on Boris.
The sooner that Act gets repealed, the better. It prevents a minority government from even trying for an election if the Opposition is minded to play silly buggers, which at the moment it clearly is.