It depends completely on the angling situation. I'd add that this question can apply to any bait, not just boiled baits.
You need to bear in mind fish stocks, session length, time of year, conditions (temps, pressure, wind direction), form (of the lake/swim), substrate/topography (spot fishing vs. area fishing), etc. etc.
No one can give you a figure of how much to use. If they do, what they're really giving you is a figure that suits their own angling situation.
I will use anything from a couple of hookbaits to several kilos of freebies depending on the angling situation (in fact I've used tens of kilos a week in pre-bating situations). If you look hard enough the carp will tell you how much you need to be baiting. As a very general rule if activity levels are low then baiting levels should be lower. If activity is rife it's time to fill it in.