I don't think you can spend more hours and get these criteria.
It's pretty rare IME to encounter interviews which are closely related to the actual work you'll be doing, mostly it seems like the main purpose is to filter out the heaps of bad/mediocre developers.
This sounds like a classic "my developer reckons he can do this in 2 hours (but never actually has)". The reality is your developer is crap at estimating.
A good developer will be lucky to produce 100 lines a day. if you doubt that check your git history (excluding autogenerated crap).
So unless the project you're setting people needs, on average, 25 lines, whatever you're setting people is clearly not a 2 hour project. And that's not even taking into account whatever hurdles you've inadvertently put in the task, strange build config, obscure libraries, out-of-date libraries, non-standard formatting, etc.
Could you tell us what the average length of a submisson is?
One task I got given a year or two back had an old version of Vue, linting rules that conflicted with the defaults of the 'usual' IDE you'd use, wanted you to create 3 new backend API endpoints and a new frontend page with non-trivial functionality. Plus they wanted units tests.
That's a 2 day job. They also claimed it would take an hour or two.
I wonder if your hiring process is skewed by the perfectionists who spend tens of hours of their submissions. Arguably that's the type of thing you guys would want to select for but it's not at all fair for those who timebox themselves to 2 hours or whatever.