Understanding ETSI EN 300 frequency hopping transmit time limit
If you have 100 frequencies, the observation period is 1500 ms. This affects test setup (The spectrum analyzer's Zero span time base, test length etc). The test will cover all the frequencies.

At any given channel (frequency), the maximum transmit time is 15 ms, not on all channels. Basically, your maximum TX time is 15 ms for any hop.