Not much. You really shouldn’t be going into grain bins, and if you you do get stuck you should call for help and shut off anything that’s making the grain move. If you have to go into the bin for some reason, there should be someone outside with you and you should have a safety rope to help pull you out. Covering your mouth won’t help for long if at all. Someone will need to put up fans to ventilate the bin. You will suffocate in a grain bin and I’ve lost friends who went into bins.

https://www.ndsu.edu/agriculture/ag-hub/publications/caught-in-grain