If you receive a full request, it usually means that an agent read it all and liked it, because their reader (or intern or junior agent) first read it all and liked it too.
When submissions are received at an agency, it's usual for interns, readers, junior agents or agents' assistants to read through them. They reject anything which the agency won't represent (so, non-fiction for agents who only represent fiction, poetry or YA books for agents who only represent crime or thrillers, and so on), they reject books which are not written well (this can include books which are sloppy and full of errors, books which are dull, and books which are incomprehensible conspiracy theories which suggest that President Bush was an alien planted by the CIA to turn America into a body-parts farm).
What's left should be the books which are well-written, interesting, and in genres which the agency might be interested in. This pile is less than twenty per cent of the original submissions pile--often FAR less than twenty per cent. I would have weeks where nothing remained from this initial cull.
Everything left over will be read through by the agents who might be interested. When they decide against a book they might not read to the end: there's no point reading pages of something they know they don't want. But if they decide to ask for a full they will read all they have, just to make sure.
Once they receive their full or partial they will read until they lose interest, and then they'll usually read a few pages more before rejecting. Almost all the people I know in publishing love their work (we have to: the pay is usually abysmal!) and are in awe of the brilliantly creative writers they encounter: they love the thrill of finding a good submission in the slush pile.
