Shaldna, you're wrong when you say that there's not much negotiating room for first-time authors: a good agent can do all sorts of things for every single one of their clients, first-time authors included.
All subsidiary and foreign rights can be negotiated in or out of the contract; royalty rates, escalator clauses, advances and marketing budgets can be specified; deadlines can be changed. There is SO much more to contract negotiations than just the advance.
And yes, boilerplate contracts are pretty standard across the big publishers: but a good agent would not allow a client to sign one of those boilerplate contracts without working to change it in all sorts of ways.
To swing back on-topic, it can take anything from a few days to a few weeks to negotiate the best possible contract: I've known some cases which took months, but they're rare. It's worth an agent spending the time on, though, as it can really change the writer's experience of being published.