A quick answer is VERY! To give you an idea, all Allan Block products have to meet an industry standard which is governed by ASTM 1372. This has a minimum compressive strength of 3000 psi. Because AB is striving to be Always Better, we hold our manufacturers to a higher standard. With that being said, the producing partners of Allan Block may be required to manufacture closer to 4000, 5000, or even higher in compressive strength. This is because Allan Block wants to provide the best quality product on the market and find producing partners who have the same goals.
Building material products are much stronger as they are the backbone of the construction industry but to put it into perspective, a half ton pickup truck can carry a load that is 33% of the total weight of the vehicle, the human body can carry an estimated 135% of the person weight, and an Allan Block has the strength to carry 7500% of its self-weight. Allan Blocks may not be structural blocks for a vertical load, but their strength is incredible which allows your Allan Block wall to last a lifetime!
Although this is a longer answer than the simplified, VERY, from the opening statement, now you know that Allan Block retaining walls are strong.