How many times?

It usaly takes me a handful of bowls to decide if I like something. Unless it clicks with me right away like Windjammer did. Old Joe krantz took about a whole 2 Oz tin before I decided it was for me. If I hate it I'll usually know after one bowl like 1792 Flake was not for me after one bowl I was like yeah the tonquin bean topping isn't working for me.

I agree with the others..
sometimes you just need to set it aside and revisit it later.
EMP, sat in a jar for 2 years. Trying a bowl here and there. Until I decided I liked it.
There are tobaccos I have found better to my pallet, in different seasons.
Heck there are a few I only like with a filter or vice versa.

This is all solid advice. As for me, I've smoked tobacco that was new to me and it only lasted one bowl before getting tossed into the bin. But it's the ones that I'm not quite sure about that I give a fair chance. Sometimes it takes a few bowls in order to find and unlock the real characteristics of a tobacco. I've found that certain ones may be ready to smoke without drying, but the flavor changed when it did dry some. Others were a definite hit right from the start. One example is Dunhill Dark Flake. It tasted harsh to me and I didn't like it at all. I discovered later that, with some drying, it was a really fine smoke. The lesson for me was to never be too quick to judge a tobacco that has potential, but to give it time. Smoke a new tobacco slowly and try to find its true flavor and nuances. In that way, you're giving it a fair try

I'll usually smoke a couple of bowls in different pipes to see if I like a blend. If I don't like it, it gets jarred until my tastes change or the tobacco improves with age. I have some 14 year old Escudo that still needs more time.

This is a question I often struggle with for myself. With the exception of two particular blends, I basically like every tobacco I’ve tried and because of that I’ve acquired quantities of some tobacco that I like, but don’t reach for very often. So I set some rules to avoid this happening. If the tobacco is a type of blend I smoke most often, say a Virginia, VaPer, or some variation of a dark fired, I decide ordering more or not within one to three bowls. Everything else I give about 1/2 of a tin (or can or bag) before I consider buying more. There are certain burleys that have a quality that I know is going to have me coming back to them often, it’s rare I run into that so I’ve usually made my decision on those within two or three bowls.

For me it’s generally one and done. If I like it the first bowl like, Presbyterian, Royal Yackt, 1792 Flake, Pirate Kake, JKP, and many of the mass produced blends. On the failed side, 5 Brothers, Sir Walter Raleigh, were decided to be not my style at all. Most flakes, rolls, and cakes or plugs are accepted by the end of the first bowl. I could go deeper but, it would be a long list of likes and not to many dislikes.

Just sying😁

There are tobaccos Zarnicky not liking immediately. I only smoking outdoors when weather is nice. During winter I not smoking pipe. After aging for 6 month, same tobacco tasting wonderful. Aged tobacco always tasting better for Zarnicky. Always giving tobacco another chance. Some tobaccos I liking right away. I can't waiting to try Dark Holler (Mirkwood). Because of cold Montreal winter, I waiting another 3 months.