I think your frequency is good, as long as there's an option to reduce that amount. That's strictly for the frequency, though.
My advice to you would be to NEVER send an email saying "hey, your trial is about to expire!" - this would be a wasted opportunity to take your users through some tasks that may be interesting to them. Tell them about a feature they haven't stumbled across yet. Explain how your product is most commonly used. Get creative with the emails and use it as a customized showcase for your user, and they might find the reminder emails (gasp) helpful!