My computer downloaded tasks for seven days, which resulted in a large list of tasks to crunch (about 60 I think).
The completion time for each Proth task was about 18 hours (according to boinc manager), which climbed to 19 hours later. I calculated, that this large list will never finish within a week.
Because I didn't want to allocate tasks which I'm not able to finish in time, I cancelled most of them (about 40 tasks).
The next morning, the remaining Proth tasks had a completion time of ~9 hours each and the finished ones only needed 3-4 hours, so I guess I didn't had to cancel all those jobs after all, right? :-/
The completion time is really weird...