You'll be best off doing what makes sense for you.
Many people check the time after a certain number of questions, often 5 or so.
I personally check the time after around every other question or when I get the sense that I'm spending more time than average on a given question.
If you feel that checking the time after every question is working for you, then maybe that's the best approach for you.
That said, you may find that you start to get a sense of whether you've gone over the allotted time per question in doing a question. If you do, then you may feel more comfortable checking the time after every other question or every few questions than you currently do.