I can answer this based on my experience with enterprise contracts sold by my company during 2012:

Unlimited bugfix guarantee:

The shortest bug fix came 24 hours after our email reporting it. The longest took about 10 weeks. On average, it takes about a week. It usually takes 24-72 hours to get a ticket assigned to someone. The time it takes to fully resolve a bug - this does depend on how clear the bug report is, how easy it is to replicate the problem, how detailed the fix is (for the most difficult bugs, we've been given profiling code to test and/or several patches that fix the bug in stages) and, lastly, how busy the support teams are.

Note: OpenERP does not automatically agree that every bug you log is a bug that is generic enough to affect most customers is most parts of the world - but I have found almost all of their decisions to be fair and well justified.

Functional Support: 2 hours.

When something is not a bug, or if you need a code example or other help on 'how' to use the software (including configuration), they will quote you the number of hours and you have the option to use them up.

The two hours is for each contract. It doesn't matter how many users you have, you get two hours a year.