I had a look at your XMLs as well as some sections of the site, to answer your questions: there is no fixed time for the sitemaps to get indexed, it depends on a whole bunch of factors like size of your site, crawl friendliness of your site (more on this later), interna/external links to the site etc.
Now it actually does not matter if you list all URLs in the sitemap, if the crawler cannot find your links then they will not get indexed - even if they were in the sitemap.
For example: the link https://www.scirra.com/forum/ i.e. the parent node of your forums section is listed in the sitemap, if you check Google's cached version of this page you'll notice that it does not pick up any sub-forums links at all (run a search with cache:https://www.scirra.com/forum/ to see what I mean)
So what I think is happening is that when Google crawler visits your forum home it does not find any of the links you placed in the sitemap (perhaps you render a different version based on user agent?) and when it sees these links in the sitemap it infers that these links are not important since they are not linked from any high importance page within the website. I randomly tried checking the cached version of some your sub-forums and sure enough, Google has not indexed them - probably because the crawler never saw them.
My other recommendation would be around structuring the sitemaps itself - why don't you use a sitemap index file (see this for more detail: https://support.google.com/webmasters/answer/71453?hl=en) rather than creating 8-10 different files in the robots file? Functionally it won't make a massive difference but it's a more elegant way of preparing XML files for larger sites.
Sorry for that wall of text - hope it was helpful!!