Welcome!
NFW means "No ForWard" -- the test will stick to the size you specify and won't automatically forward when it might otherwise. Note: under certain circumstances it may forward, for example, there are minimum criteria for test size (6 MB download, 1.5 MB upload).
Sorry, right now 100 is the max. If you extend the time between each test you can get 100 days.
One of the reasons it's set that way is because I don't want people mindlessly forgetting about the test. Here's what I can do instead of that limit. The next version can allow longer duration but will periodically email you to remind you that it's running. If you've set a test over a month long duration, it could email you the log once a day.
It's actually built in to allow more time... but you have to know that to make it work that way. I'll PM you a link.
I'll work on giving you more options to extend the time in upcoming versions.