Originally Posted by
RCyyz
I'd say that 2h would be the minimum comfortable connect time from T1 to T3. If you're looking at a sweating-bullets-I'm-not-sure-if-I'll-make-this-flight then the number would be a bit lower. With 2 separate PNRs you do need to check in at the CX counter so you need to be mindful of minimum check-in times. (i.e. If you don't have your BP at T-x minutes prior to boarding you might be SOL.)
I would be more worried about the baggage cut-off than the minimum check-in time. They can always override the check-in deadline or you can do OLCI but they probably won't take your bag if you're a minute past the deadline.
I'm not sure who flies from LGA to T3 these days. I do know that you might make a 1.5h connection on 1 PNR if you take the AC flight to HKG instead of CX from T3.
The minimum transfer time at YYZ for AC US->Intl is 1 hour. The whole thing would be 20min of walking with no baggage collecting and only a cursory passport check so you don't need to pad that unless you're worried about the LGA flight being delayed.
YYZ baggage handling is not the swiftest and LGA flights aren't the most timely. On two separate itineraries you want enough extra time that you don't have to worry even if your flight is late and your bags take a while.