I took phpBB and added a resource management / calendar function for the company I work for. It took over a hundred hours. One key thing to remember is that you can't just make the "final product". In many cases, especially if you're adding new features (and that's sort of the point, eh?
) you need to add supporting code as well. For example, in my calendar addition I needed about a dozen new admin screens. I needed screens to add / edit / delete resources, resource types, billing rates, clients, client locations... and so on. If you're willing to take shortcuts, you can skip a lot of the data validation for screens that "only you" (i.e. admins) will be using, making the assumption that an admin knows what they are doing. But anything that is presented to the final user needs to include error checking, data validation, and all that good (but tedious and sometimes boring) stuff.
That being said, phpBB makes (in my opinion) an excellent base for this sort of thing. The database is well designed, the code is well structured, and it's nice to not have to redevelop the registration / login / profile sort of screens. Even if you don't use a bit of the actual forum code (I did but my company is not actually using the forum portion, just the calendar / scheduling modules) it's not a bad thing to have. After all, just about any type of project needs a way to discuss things, right?
Without a more specific list of features, it would be difficult to give a true estimate of hours required for your project. I finally made an admin "template" php file where I removed a lot of the hard-coded stuff, leaving me to only edit the SQL code in order to create a new admin file. That saved a lot
of time and debugging. Each admin file needed it's own templates (one to list and one to add / edit data) but that's quick too. I got to where I can develop a new admin screen in about an hour with these changes.
Other screens, depending on their complexity, may take up to a week to design, develop, test, and release. It's a good idea to know where your project is going before you get there, in order to make sure that all of the features you require are designed into the database, and that you have a good workflow (story board) established before you start coding.
But you probably know a lot of that already.