How strong is it?
That depends on "which pachanoi". It is highly variable.

How do I know if I the San Pedro I have is strong or weak?
Ok, so this dude sold you some San Pedro. Do you have the strong kind or the weak kind? Neither you nor I have any idea. Probably neither does the dude. There are some suppliers with a track-record of selling "effective" San Pedro. If you are risk-averse, see if you can find someone who has been in business for some time and ask their other customers.

So how do I know how much to take?
Now you're sitting there with a cactus cutting in your hand, intending to commit felonious cactophagy, and are about to add aggravated weenie-cide unless I can give you a straight answer on how much of that cactus in your hand you should eat. The "maximum safe dose" of mescaline, according to Ott, is 1000 mg. Assume the worst (best) about your cactus, that it is H&B's 2.375% (dry), the most powerful pachanoi known to science. You would therefore not want to take more than 714 gms (fresh), or about 1 1/2 lbs. The threshold dose is about 150 mg, so you'd need at least 107 gms (nearly 4 oz.) to get any effect in the best (worst) case. If you got the wimpy stuff you will need to eat at least 2.5kg to get the least effect. To get the maximum safe dose from the wimpy stuff you need to eat ~17kg; probably about one-fifth your own body weight.

I got the wimpy stuff, now what do I do?
Let the rest of it callous off, root it out, plant it, and join the rest of us in discovering what this organism wants from life. It may or may not be weak because of lack of genetic potential; nobody really knows. This is not Safeway, this is a living organism we are approaching and attempting to enlist. Negotiations may be in order. If nothing else, the flowers are gorgeous. Join your local cactus society, or a net mailing-list and expand your horizons.