I would reduce those 15-20 mins even more. Example from few weeeks ago :
we were doing exams and we were finding strategies/videos from many different sites. so i found the lvl 117 video for example, and i send it to my guildmate. another one wants to try that too, but now i can't send him my strategy for 30 minutes, but have to ask the first one to send him the link. Now a 3rd guildmate shows up and either first or second go off, so that one has to wait 30 minutes till i send him my link.
Internal guild mails should be way faster and imo, with no restrictions. Mails that go out of guild could be time limited cause of spam issues.
Or, make it possible for guildchat only to be able to copy/paste from the text.