Has this ever happened before?
Yes, and no.
Officially, on paper, yes. 1979. Newfoundland. The Liberal executive chose Donald Jamieson as the Interim leader, and he lead the party through the 1979 elections. Jamieson didn't do that poorly, gaining 3 seats and 3%.
In 1988 the Manitoba NDP was in dire straits when on March 9th, the budget failed a vote in the legislature, and the government fell. The Premier resigned as party leader. March 30th saw a leadership election which Gary Doer won, and April 26th saw the general election. A ridiculously short timeframe.
The closest comparison we have, however, is 2015, and Wildrose. Danielle Smith quit the party on December 17th, and the election would occur May 5th. 139 days. Brown's resignation comes early this morning, the 25th of January, and the election is on the 7th of June, 133 days. Though clearly rushed, Wildrose was able to hold a leadership vote within 101 days (This would work out to May 6th 2018, a saturday) That election would select Brian Jean as leader.
There are, however, many more stories from parties in provinces that ended up acclaiming someone who originally was selected to be just the Interim leader, the 2011 Federal election and Liberal Micheal Ignatieff for example.
Given history, chances are that unless the PC Party announces tomorrow that they will hold leadership convention before June, whomever the interim leader is, is very likely to be the permanent leader.