Let's rephrase your question to be more accurate. "How close to sin can i get without actually sinning?". That's usually the root of the question. And while it's true that it may depend on the individual couple, the problem with that is you have no way of knowing without taking yourself there.
Second base, though, would still be a clear line crossing. It is simply foreplay. Which is what people do before intercourse. Not to mention it is an obvious temptation, which we are told to flee from.
When dealing with temptation you are better erring on the side of caution. If you miss out on a few things, that maybe would've been ok for you, you're not going to regret that in 20, 30, 40 years. On the other hand, if you go too far that can be a guilt that, literally, lasts a lifetime. Trust me, i know.
The trick here is also remembering that what one may handle, doesn't mean the other can handle it. If, for you, cuddling is fine and provides no temptation, that may still be a temptation to him. So the stronger has to stop themselves at the others boundary and respect it, not push their partner to go farther than they can handle.
But, basically, if holding hands provides too much temptation, then don't do it. And don't try to go as far as you can. Because (and again, trust me on this) once you try to dance on the edge of sin you Will cross over. You Will fall. As Christians our lives are meant to be lived holy and blameless, not seeing much we can get away with and hoping we don't screw up.