For any framework, API, SDK, library, etc., getting "hello world" working is easy, but knowing all the ins and outs well enough to take advantage of the strengths and avoid the weaknesses takes experience.
There are levels of facility:
- Bare minimum: You can understand conceptually how it works, you can make minor tweaks or bug fixes to an existing instance. You have to consult the documentation for every use of the API. This takes one-half day to two days.
- Productive: You can start new projects, add new features, design solutions. You have to consult documentation for some tasks, but others you can do from memory. This takes about one to two weeks of working with the framework.
- Fluency: You can extend the framework, and integrate it with other libraries. You've not only learned all the advanced API, but a bunch of clever ways to use options in the basic API. You've read all the documentation and you're writing a cheat-sheet for the other members of the team. Reaching this stage takes at least three to six months.
This assumes you have enough foundation knowledge to come up to speed on the architecture. You're in a good position given that you've worked with the concepts in another MVC framework.
I'd be careful of billing for time when you're not even at "bare minimum" yet with this framework. Some clients are not comfortable paying for your time to come up to speed on an API (unless it's really specific to their project).
If you have a good relationship with the client, you might be able to talk frankly with him and come to an agreement about how much ramp-up time he'll pay you for and how much is your responsibility.
I think most programmers stretch the truth a little bit about their level of productivity with the unfamiliar API, and then hope to work with enough privacy that they can get the job done without showing too many missteps while they learn. Then pad a little bit the hours reported for other tasks, to account for the ramp-up time. Is this dishonorable? It's a question of degree; adding 10-20% to the time for overhead probably won't get any objection, but tripling the hours wouldn't be fair.
It goes both ways, too. When you work for a salary, an employer may expect you to work overtime occasionally, so your total hours averages 10-20% over the "official" 40 hours/week. But expecting double or triple hours per week on a regular basis would be ridiculous.