Different map providers have different abuse-prevention mechanisms, but many limit tile downloads from a single device to 10,000 tiles per 24h period.
Even for premium maps (except IGN), such an abuse-prevention is used, and MyTrails will be able to download only 10,000 tiles per 24h period. The limit will be reset 24h after the first tile was downloaded, not 24h after the limit was reached.
Note that at latitude 45° for zoom level 15 (the zoom level used for 1:25,000), each tile covers about .8km², so 10,000 tiles represents about 8000km², so the most popular offline subscriptions can be entirely downloaded without hitting this limit.
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