if you stay up to 20 MB, everything should be fine, but, as always, the less you use, the better ;-).
as far as I know, it is not documented when memory warnings are triggered, probably because this is a detail of the implementation, and this can change at any time.
In any case, you should not pay too much attention to numbers (you do not know other processes related to memory consumption (safari caching pages, applications in the background using memory), just be a good ios citizen, make sure that you do not leak memory everywhere , and your application will work like a charm.
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