You didn't mention the distance involved.
I had a three-hour journey each way to my parents when they needed help, so helping can easily take all day if this kind of distance is involved.
How you tackle this is hard to say - it depends on whether an almighty row with your husband would clear the air or lead to a divorce!
I would try going with him and helping first. If he or MIL don't want that, your next option is to sit your DH down and quietly explain that you are worried that working full time and helping his mum every weekend is affecting his health as well as your relationship.
We latterly did my father's shopping once a month, filling his freezer with meals I cooked while at his place and put in the freezer, but he did have a home-help doing some cleaning and his washing, and the district nurse in to help him take a bath.
I assume as you are both in your 50s the MIL is in her 70s, so this could go on for the next 20 years.
How does you DH get on with his brothers? If they are fond of each other, I would suggest they divide the tasks to be done for mum between them, and simply tell her that this is the way it is going to be, as none of the three of them can do it all on his own.
If your MIL is in the terminal phase of an illness then obviously that would change matters somewhat, but it does not sound as if this is the case.
However, if your husband will either not discuss matters after the row, or refuses to see your point of view, you really only have two options: put up with the situation because you love him and pick up the pieces when he cracks up (which is sadly likely to happen) or sit down and work out whether this is the end of your marriage.