There is no set limit or guideline on volume.
But really volume should not be the issue. There's a huge number of factors that is taken into account to determine if a message is spam... too many to discuss here. But you can assume basics: the content of the messages, the volume per user in a given time period, the content variation per user, has this app been flagged as spammy before, by how many users, etc. I would say its reasonable to assume your test user is probably going to be triggered as spammy because you're likely using it far far more than the average user would.
So, in short: it depends entirely on what exactly you're doing every 15 seconds.
Tip (although I cannot verify it): if you are trying to batch send a message to a number of users at once, without them very recently contacting you, you're probably gonna be flagged as spammy.