How strong was Frankenstein monster?

The answer depends on which Frankenstein Monster you mean.

In the original novel, he (the creature/monster) was eight feet tall and, apparently, fully proportionate with his height. That would make him much stronger than any human just by virtue of his immense size and muscle mass. He is said in the novel to also be superhumanly fast and agile.

The old Universal movies are probably the best known rendition of the Monster (square head, bolts on the neck, etc). These films spanned 1932-1948 but continuity between them was loose and inconsistent at best. However, they all are supposed to feature the same Monster.

In the Universal movies, the Monster's strength varies considerably. It begins with Boris Karloff's original Monster, who is perhaps two or three times as strong as a normal man, though incredibly tough and nearly impossible to subdue, hurt, damage or destroy (besides the basic makeup design, this is perhaps the one consistent point throughout all Universal's films...he gets progressively dimmer as time goes on, but gets much more physically formidable and frightening).

The next big jump in strength has to be Bela Lugosi's Monster in Frankenstein Meets the Wolfman. The Monster, upon receiving an electrical recharge, casually flings heavy lab equipment across the room and beats the snot out of the Wolfman whenever he manages to get a hold on him.

Finally, in 1948, there was Glenn Strange's supercharged, robotic giant in Abbott and Costello Meet Frankenstein. At the start of the film, the severely weakened, near comatose Monster can still carry a couple hundred pound coffin single handed. At the end of the film, he's fully recharged and not only moves much faster than before but rips a huge iron gate out off its hinges with one hand.

Of course there are literally dozens of other depictions of the Monster, all of which are, at minimum, superhumanly strong.

As far as toughness goes, however, the Monster is near the top of any list. In various movies he has withstood multiple gunshots to the heart, blazing infernos, explosions, being immersed in 800 degree sulfur pits, burial underground for years, brain transplants, freezing and drowning. It should go without saying that he survives electrocution with no problem.