Our son's homework varies enormously between subjects. A few give regular homeworks once or twice a week, which is fine because they're usually relatively short/easy to do, such as a worksheet or learning for a progress test. Others are a pain in the neck as he can go weeks with nothing and then gets a huge assignment which is going to take several hours at very short notice, i.e. for 2/3 days time not even over a weekend. Makes it very hard to plan for. And yes, this is all as per "show my homework" so it's not as if he's forgetting about it to the last minute.
I've been drumming it into him since year 7 that he has to stay on top of things, revise topics as you go along, etc., whether anything has been set or not, all to avoid the last minute panics as the tests/exams get close, but the teachers really don't help with that mind frame at all, as the teachers themselves seem to go into panic mode as the end of year tests loom closer and set vast amounts of homework/revision etc in the last few weeks, which is really hard to manage, and completely unnecessary after months of giving out very little homework!
To the OP, I wouldn't stress too much, but I'd certainly keep an eye on it and maybe see if you could work with your child more, i.e. test them yourself occasionally from their books, or buy revision/practice manuals early and start working through them.
Our son was very reluctant to do anything not set as formal homework in his earlier years, but we've broken that down and now he'll do any "extras" we ask him to, but we're reasonable about it, only do it when he has nothing else on, and we do it with him rather than make him sit and do it on his own. We just chipped away at his attitude of not doing anything not formally set, and after a couple of years he's amenable now, more than anything because he's seen the benefit of staying on top rather than dropping down and having to work even harder to catch up again. A couple of low moments along the way have helped change his mindset. In year 7, he got a couple of poor (for him) year end test results and luckily for us they were in subjects where the teacher hadn't set much revision homework and he knew himself that his revision was inadequate. In year 8, he got a poor score in Maths, his best subject, in the year end test, of just 67% when he's usually 90%+, which set a rocket up his bum as the teacher had set virtually no homework during the year and had only given a token amount of revision homework shortly before the test. Again, a demonstration to him that regular practice and revision was essential to keep his scores high.
And yes, keep an eye on their scores/grades etc to make sure they're keeping up to a high standard and come down heavily at any drops.