It's surprisingly difficult to find velocity and altitude profiles for a shuttle launch, but I did hunt down a few figures.
The SRBs finish and are dumped at around 2 minutes, and at an altitude of around 140000 feet. You are probably going to be a having a hard time of breathing at that point.
As others have said, it would probably be a bad thing if you tired to hold your breath into vacuum. I guess hyperventilating for a while to get as much oxygen into your system as possible would be one approach to surviving longer.
Velocity is also going to be an issue if you're not Superman, the Shuttle is doing something like 100mph shortly after it clear the launch tower, 1000mph after about a minute, and 3000mph, more or less, about two minutes after launch (when the SRBs drop off). You'll need a firm grip to hold on, assuming air friction hasn't warmed you up too much.