It's not a silly question! No questions are
I think it can be compared with say an artist, This is Martial Arts after all.
An artist can make a painting or a sculpture and show it to someone and they might say "I think you need to give it more time". Its not a specific question about the amount of hours or the exact time. It rather means that you have to put in more time in general to get better.
As for how much time, it's probably "as much as possible". I mean wouldn't it be awesome to train 18 hours a day 7 days a week? But who can do that, practically nobody...
When it comes to the term "Mat time" I think people refer to experience on the mat, more than the actual exact hours training spent there. It's not the same "mat time" to lay on the mat for 4 hours or to train hard for two hours. Its not about the actual "Mat" its about getting the training in.
If someone says you need more mat time you probably need to add a day a week of grappling and wrestling training, for example.
Mat time could also refer to time spent on the mat actually competing and not just training. I think it depends on under what circumstances its mentioned.
Just my 2 cents...