They won't let you take more than a week off after orientation.
Also, you need to understand that you will make a gamble regardless. You could make it through orientation and then fail out of the OTR training w/ your trainer for any number of reasons. Then after you finish 5 weeks of OTR training you get your own truck and hit something day one...well, you could be out of a job w/ TMC day one depending on the situation.
So, again...it will be gambling regardless of when you put your notice in.