This app was mentioned in 2 comments, with an average of 2.00 upvotes
It's wildly variable depending on type of tea, brewing method, your preference, etc.
As an example, I use 6-7 grams of rock wulong in a 150mL yixing pot, with steep times around a minute to start. When I make a pot of my utility black blend, I do probably 30g in 1.5L. At this point I can do it mostly by eye so I don't have the exact numbers handy.
As a beginner, forget how much tea is in a typical teabag; you'll want to use a larger volume (and make sure it has room! No steeping spoons!) but you can amortize it over multiple steepings. Many of the nice tea shops give suggestions about mass and time for their products, so you can use that as a guide.
I use this timer:
https://play.google.com/store/apps/details?id=com.rdconcept.android.perfecttea
It allows custom timers that include the water temperature and amount of tea, so you can keep track of what you like.
tl;dr: Experiment. Start with "a lot" of leaf.