Anyone who has tried to pick up a snail crawling on a smooth surface like a sidewalk or glass may have noticed that snails can have quite a tenacious grip. One way to measure how strongly a snail can hold onto a surface is to measure how much weight the snail can lift.
To collect some relevant data, I put together a crude apparatus consisting of a small plastic bottle taped to 2 glass microscope slides. I can increase the weight of the apparatus by adding objects, usually coins, into the bottle. The snail is placed on the upper slide. After it attaches its foot on the glass, I hold its shell and lift it up, at the same time starting a stopwatch. I selected 5 seconds as an arbitrary minimum time necessary for a lift to count as successful.
This Cepaea nemoralis carried, in addition to the apparatus itself, 3 quarters and 1 dime, or a total of 41.3 grams for 39 seconds.
That weight will be more meaningful once I express it in relation to the surface area of the snail's foot. I haven't had a chance to do that.
Keep in mind that a snail does not not actually hold onto a surface using muscle power; its grip results mainly from the functioning of the sole of its foot like a sucker. Parker* wrote in 1911:
As means of attachment snails secrete a bed of mucus, and use the foot as a sucker. Both methods are commonly employed by the same species, but in a given form one method is usually developed much in excess of the other. For instance, in Helix pomatia, Limax maximus, and other allied species, the moist surface of the expanded foot will stick with some tenacity to glass. But if such an animal be allowed to creep its length over a glass surface and thus spread a bed of mucus on which it can rest, it will be found to have multiplied the strength of its attachment many times. The mucus adheres to the glass and the surface of the foot to the mucus very much more powerfully than the foot alone can adhere to the glass.I have noticed it is somewhat difficult to obtain reproducible results. There may be some habituation involved. If the snails are picked up too frequently, they appear to start letting go off the surface more easily.
*G. H. Parker. 1911. The mechanism of locomotion in gastropods. Journal of Morphology, 22:155-170. pdf