How firmly, or how long, shouldn't matter with a shutter in good condition. It should either trigger, or not, and should complete the open/close cycle regardless of how you press the shutter release. As long as you're not in "B" or "T" mode, how long you press the release for shouldn't be an issue.
I'd try a few test fires and see if it's working reliably, or sticking.
It won't be a super accurate way of checking, but you can use a program like Audacity to record the audio of the shutter cycle, and measure the length inside audacity and see if you're getting consistent results.
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