Hi Davey !
Any kind of meditation a day is good, even if it is just a quick 15 minute meditation because you lack the time for more.
Twice a day is good, and some prefer only meditating early in the morning before work, others, like me, just meditate in the evening.
The free course works just as well as the TM method, I recommend listening to the course every now and then, especially when you are new to a meditation method like this. It is full of information that makes more sense after a week or so of meditation practice.
Overmeditation is about release. You release old emotions and memories that have held you back your entire life ( for instance, you hate swimming so you never go swimming with your kids, and feel guilty towards your kids, but you really can't go swimming ), but when you meditate a lot, you will release a lot of emotions at the same time. Some people might feel uncomfortable with that, because they might be racing through your mind waiting to be released, but you might "cling" to them, wondering what is going on that you suddenly remembered this embarressing moment from 20 years ago where your swimtrunks ripped while going down the waterslide in a filled pool ( this could be one of those "Aha" moments

).
If you have a truckload of these kind of memories flushing your mind after having done extensive meditation, it can be quite overwhelming.
Basic rule: As long as you feel good, keep it up, when you feel troubled, tone it down

.
It can creep up on you unnoticed, so monitor yourself and your emotions.
You will grow into it soon enough. The more you meditate, the less emotional baggage, the longer you can meditate, the better you will feel, it is a process.
Have you downloaded the free 10 minute LifeFlow trial yet ?