Tab, in my opinion, is great for getting the general pattern or notes of a song, but pretty poor at conveying real musical information. It would be pretty difficult to sight read a song notated in tab that you had never heard before, as tab does not provide note lengths and expression symbols in the same way that notation does. I have always used tab to learn the progression of notes, and then set them in place by ear.
That being said, tab can provide some timing aid. Usually, the dashes that make up the stave can help you to identify what beats, semi beats, or even quarter beats look like in the context of the bar.
Using a very simple example to illustrate, in 4/4 time,
Indicates crotchets, while
Indicates quavers. So for a bar with 16 dashes, a note with 2 dashes is a quaver, and 4 dashes is a crotchet.
This is not very easy and quick to work out when sight reading, hence why tab is better for reference. It can provide a basic idea however.
Also, as slim said, many music books provide tab beneath the score, so you can see the nature of the note directly above the tab note in order to get the timing.
I've probably explained this in the longest and most complicated way possible, sorry about that. Hope its given some help though.