There isn't really a specific, global threshold. The context, content, and length of the message can affect how disruptive it actually is.
If you feel someone is borderline spamming the mic and/or is being disruptive (which I guess is every 15 to 20 seconds for you), here's what you should do:
- Politely ask them to say their message a little less frequently.
- If it continues, ask them again. No more than twice is necessary.
- If it's still a problem, don't confront them. Instead, get an Admin/Mod's attention (typing /admins will give you a list of in-game Admins/Mods; if there are some in-game, you can message them by prefixing a team-chat with @ You can also message them over Steam), and let them make the decision.
- If no Admins/Mods are available, report the player with !report or on the forums, and then mute them yourself (Esc > Mute Players).
Following that will either resolve the problem, or resolve it for yourself and in some way notify an Admin/Mod. It'll also prevent any unnecessary escalation. Either way, you're happy.
EDIT: If the Admin/Mod makes a decision you disagree with, you have a couple options, none of which will get you in trouble (for the most part):
- Report the play anyway via /report even though the Admin/Mod said it was fine
- Mute the player yourself
- Report the Admin/Mod if you think their decision was completely incorrect
- Get another Admin/Mod and hope they'll decide in your favor (don't do this unless you strongly disagree with the previous Admin/Mod's decision; doing it often will likely get you removed from some friends lists)