It depends on how certain you want to be ;) However, you have more than just the Skytrain option, according to Wikitravel:
There are a number of ways to get into town from the airport. Prices and directions below are for getting into downtown Vancouver.
SkyTrain - The Canada Line  provides the only direct rapid transit public service downtown, in 25 minutes. The fare from YVR to Vancouver is currently $8.75, which includes the two-zone base fare of $3.75 plus a $5 surcharge (the "YVR AddFare") incurred on cash fare tickets purchased from vending machines at the airport. The $5 surcharge only applies on trips starting at the airport, not on trips going to the airport. It does not apply to prepaid tickets including DayPasses, FareSavers, FareCards and transit passes. To legally bypass the AddFare and save $5, simply buy a FareSaver ticket or day pass at the PharmaSave or 7-Eleven located within the airport.
Taxi - Taxis line up just outside the baggage claim areas. A taxi ride into town will cost about $25-30 and should take under half an hour. All taxis that serve the airport are required to accept credit cards.
(hehe) Limousines - Limojet Gold offers comfortable sedan and limousine options for getting into town. Rides into the city centre cost $40-55 depending on where you are going and whether you are in a sedan or limo.
I'm of course assuming you really want to take the sensible option of the Skytrain, and merely want to know how much time to leave. Your best bet is probably to just keep an eye on twitter/the alerts page and see how things are going with a couple of hours beforehand.
Personally I'd assume it's going to be ok, panic a few hours before, and have a back-up plan with taxi numbers at hand - but odds are it'll work out in the end. Just be sure to have that back-up if for no other reason than to give yourself peace of mind. And please don't actually take the Limo ;)