Depends on the event in question. For polls asking, "do you like such-and-such a candidate now?", the event of "now" happened as soon as the question was asked, so they can be fairly accurate measures of how someone is perceived. The same or similar polls have also spent the last nine months or so telling us that Trump was on course to win, and win big, in the Republican race. So... that turned out to be pretty accurate too.
At the moment, the polls show that Trump is disliked by many more people than he is liked by, and also that, since the polls started, his ratings have essentially remained constant. Clinton's picture is similar but less polarized. Once they start going head-to-head, it will be interesting to see how Clinton copes, although she's managed just about to hold off concerted opposition from her left.