When tying on tippet, how much is recommended? I heard something like 18 inches but I am not sure. Thank you very much!

I would reccomend a minimum of 18 inches. In some cases you might want as much as 3 feet or even more, but I'd say 2 feet is a good length to start.

I agree with Jake... around 24-30 inches is usually what I tie on and when it gets down to around 12 inches I will replace it.

Remember,
The longer....the more it'll stretch. For this reason...the next section up from the tippet --> lengthen it a bit too....as to not create a large gap between light and stout diameter sections...

I usually add around 18" of tippet material to my leaders. When the tippet material gets shorter than 8" I will cut the tippet off and add another 18". Just my $0.02 worth.