I have read the answers to a similar question about travelling from Toronto to Niagara Falls. However that question was asked a few years ago and the train systems and traffic congestion in Toronto may have improved/deteriorated since then.
The length of time it will take us to get from Toronto Pearson International Airport to Niagara Falls will be roughly 4 1/2hours, by taking the UPExpress to Union Station, then a ViaRail train to Burlington and then a bus to Niagara Falls. No doubt hiring a car would be time saving but I don't wish to be caught up in peak hour traffic.
My flight gets into Toronto at 3pm.