Let's say you're in the planning and writing stages of your thesis — this could take several months, top to bottom — and you don't have regular meetings with your supervising professor.
Or your professor takes a four-month trip to the other side of the world while you're in the midst of writing (this is arguably when you need your supervisor the most).
And then you send your supervisor a bit of your thesis, in order to get some comments on it; sort of a general "how am I doing?" kind of thing.
But it takes your supervisor a full month to comment on it.
And when you do get comments back, they're of the "yeah, I don't think we're going to be able to defend this in the time-frame you initially suggested" variety.
Which means you're going to have to pay tuition all. summer. long.
Even though you're flat fucking broke.
As soon as you start to realize, "Hey, maybe my supervisor is sorta just leaving me out to the wolves," it is time to sit down with your supervisor and have a serious god damn discussion about where your research is going.
It's not that I want my supervisor breathing down my neck as I type every word. But a little "hey, how's it going?" or "could you send me a sample of what you have so far?" or "what stage are you at?" could be really, really helpful.
If you want this, and you're not getting it, ask for it.
Learn from my mistake, people.