I guess it depends on the size of the kit. For me if we're talking a wilderness PSK about the minimum I'd go with is 20'-25'. Much less is asking for trouble IMO. 50' is not overkill but I don't think I'd often need 100.

My main use for cordage in a PSK is to build a shelter, normally a large mylar blanket into a lean-to or plow-point configuration. My AO is the forests of ID and MT, so most of the time I'll be pitching a tarp with a ridgeline running between two trees. Since God doesn't plant them with ridgeline spacing in mind I sometimes need more cord, sometimes less. If I have trekking poles along it's a bit easier and I don't need a ridgeline, just guylines.

So if I'm pitching a shelter and then to lash anything that 25' of cord can start getting used up.