It all depends on the requirements thats why everything is adjustable. Bigger HDD are cheap nowadays so we normally set it to the highest possible setting depending on how many cameras and the amount of days required and the size of the HDD.
Nobody has a crystal ball (not one that reliably works anyway) so nobody can predict when they are going to need to look at theyre CCTV to see what happened in the past. In most cases its after their car got broken into, or they were burgled, or a neighbour was burgled, or an incident that happened that day or within a few days. So ideally you need good recordings for the first couple of weeks at least.
We rarely use 25fps on domestics in most cases 10fps will gather enough info, I drop the bitrate if we are not achieving the days required on motion detect. At home I have two recorders a cheap one recording the sub stream and the premium one recording the main stream. I get about 2.5 weeks on the premium, I figured if I hadnt noticed something that happened anytime over two weeks it wont be that important. However, just in case, the cheap NVR which still records better than my old analogue system for 2 months. It works well for me I might try selling that idea to my customers sell two recorders = more dosh. As a bonus to security one recorder is in an out building so it would be difficult to steal both before the Police arrive if I were burgled, so its win win