How long does now last?

Now as in present, at this moment.
Does it exist at all, the now?
When I think about it I come to the conclusion that when I have a thought, now, it has already gone by the moment my thought ended, thus being part of (my) history.
I understand that this appears to be (probably even is) a vague question, maybe even unanswerable, but still it is something that comes to mind every now and then.
Fluther being the site it is (and populated with the Jellies it is) I thought: “Let me throw this one into the smack”.