How long would you be in jail for underage drinking?

Depends.

In Canada it tends to often (In BC anyways) be treated like a ticket $150 and less than a penalty for driving without a seatbelt.

In the US it is more hazy and you may go to jail or licence suspension.

In all the US seems to penalize the minor while Canada tends to go after the establishment or person supplying the alcohol with (liquour licence suspension, large fine, jail time)

Kensei: You can probably expect an 8 hour trip to the drunk tank.