I've been working with WordPress almost exclusively for the last four years and I would go for at least three days work, assuming everything worked flawlessly.
My experience comes from having to work on projects in different ways; I've worked alone, with freelancers, and supported after freelancers worked alone. In every case, I have been the support after and I know there's a lot of lazy people out there because they want to rush through the job and I have often rebuilt the entire site in order to fix huge issues that weren't found til after (Q&A was not something my former employer indulged in ).
Day 1/2 -
I'd setup in a dev environment first and test, including all plugins, some basic content and show it to them. I've never worked on a site where they are 100% happy with everything as it is, there's always some tweaking or extra configuring.
Are they adding content, or you? This can be extremely time consuming depending on the client.
Ensure content is up and verified.
Day 3 -
Transfer content and full site to the live server, as it is all verified there should be no issues from them. Check each page, test, test, test!
Now show them how to use it!
(If they are putting up content then you need to show them how to use it beforehand).
It doesn't take long to setup WordPress in a basic form, but all the fiddly little customisations can take a while, and content is always a pain.
You might use a form plugin and then find it doesn't work correctly on the live server (happened to me many times), or maybe the live server is missing something so that could be another few hours of troubleshooting (also happened many times).
I'm guilty of under charging slightly and I would probably give a charity a discount, just be realistic about how long it will take even if you under charge as it will affect the next job and if you get delayed by three more days the client may get frustrated.
My Dad was a software developer and IT manager for some pretty big companies over the course of his career and was well known as being thorough and honest, he always said if you're not sure how long it will take add 50% more time (I personally think that 50% is excessive). It doesn't mean you have to charge for 4 or 5 days, it just means you are prepared for it.
Remember there's 24 hours in day if you need them, not 8
Hope this helps a little