As with many things on Stack Exchange, one should ideally protect questions which need protecting as one comes across them in one's normal usage of the site, rather than going out in search of questions that need it and protecting many at once.
If you come across a question which
- has attracted multiple low-quality answers from newish users
- isn't already protected
- is old enough to be able to protect
then protect it. (If the third criterion isn't met, and the first is egregiously bad, you might consider flagging it for a mod to protect. I've done this many times on sites where I lack the rep to protect.)
But in the normal run of things (at least on most sites, including the only site where you have the power to protect), you're unlikely to come across many such questions in quick succession. Even as a mod on that site, having my attention drawn to most of the low-quality posts via flags, I don't think I've ever had to protect more than two questions within an hour. So I doubt you'll ever need to worry about a rate limit - and if you do, it might be a sign that you're protecting too many questions.