As mentioned above several times, there is no rule of thumb other than general guidelines from the manufacturer for each flavour. Some flavouring houses produce more concentrated concentrates than others ... for example ...
Flavourart, Inawera, Lorann's - individual flavours can commonly be mixed at 0.5-5%, depending on the flavour
Capella and The Perfumer's Apprentice - generally 5-10%
FlavorWest, One to One - generally 10-20%
One shot, ready-mixed flavour blends like Pink Spot and T-Juice are generally mixed at 15-20% or more. We've seen many more entries into the one-shot market in the last year or so.
Most flavour houses offer different types of flavouring - individual flavours like apple for example and blended flavours like apple pie and custard. This partly explains why there's a range of recommended %s within a single flavour house. You will probably use a higher % of a blend than an individual flavour. Also, some flavours are naturally stronger than others - peppermint would be a strong flavour, peach would be lighter and more subtle. Stands to reason that you would use less of the stronger flavours and more of the subtle when combining them in a recipe, otherwise you will get one flavour dominating. (I'm not recommending peach and peppermint by the way.)
When mixing complex recipes, which might use up to 10 individual flavours, it makes sense to use the more concentrated brands, otherwise the total flavouring of a mix can creep above 20%. I'm not totally averse to going above 20% on occasion but reaching 20% tells me that it's worth reviewing that recipe. No problem with one shots because they've been "watered down" with PG to make them more profitable , but 20% of highly concentrated flavours is way over the top.
When I'm mixing a juice using individual flavours I'm in the same ballpark as
@Mr Numpty , when I'm making up a juice from one-shots then it's 15-25%.
Using too much flavour is both counter-productive in improving the flavour and needlessly expensive, it's also likely to be less healthy. Flavouring is the most costly element in any mix so if your primary motivation for DIYing is to save money then you'd be daft to use too much.