It's best to have your relationship in writing. After all, this is a business relationship, and affairs that involve money should be spelled out on paper. Agents can offer one or multi-book contracts. If your agent doesn't offer contracts, at least get a letter of understanding so that the important terms of your relationship are spelled out. That's the most difficult question to answer.
Typical book offers range from zero to one hundred thousand dollars.
If the prediction is too low, authors begin to lose faith in the agent's professed knowledge about publishing. The size of the advance may not be the most important element of publishing contracts, anyway. Under certain circumstances, the royalty schedule may be more significant than the advance. For example, a high advance coupled with a stingy royalty rate on a successful book may be worse than a low advance and a higher royalty rate. Most royalty rates are on a sliding scale, escalating as the sales of books increase.