There is no such thing. Every environment is different and every customer has different needs. Of course in an ideal world you'd have enough resources to run all the searches on time but in reality, especially if you have clueless users who schedule millions of searches on the top of the hour it won't work. It's up to you and the characteristics of your workload to decide what is acceptable. Some customers need a strict adherence to the schedule (and might even set up separate search heads just to do schedules reports uninterrupted) whereas others are ok even if a search is delayed quite a lot.
So YMMV.
