It totally depends on your fitness - also some of these mountains are easier to climb than others. There's a challenge called Ramsay's Round, where fell runners compete to do the highest number of Munros in a day. I think the record is 23 Munros in 24 hours, or something crazy like that! Some people do all 282 Munros in three months. There's a really nice film on YouTube about a guy who did this. But I don't think it would be much fun. I prefer to savour the experience! Ideally I'd like to finish mine (known as "compleating") in 2020.
I also have a friend who did 157 Munros in a year, because she was so desperate to get them all done. She did loads of them in terrible weather, without seeing the views... she is an amazing tough lady though!