That's such good news for your Mom. Most likely they will monitor her for the rest of her life but that's not a bad thing. If some little thing crops up again, they'll be right there to deal with it. I wish all other women could be checked closely and avoid this type of cancer. A friend of mine was diagnosed, stage 3 and it's been 8 years. She's not in remission, still doing chemo but she's active (traveling all over the world) and enjoying life.
It sounds like she has done really well on her treatment, I am sure that is a relief.
There is no clear answer to your question as there is no reliable way of determining what the future will hold. As there is no disease at this time, it is quite possible that your Mum will remain in complete remission and have a full and normal lifespan. There are many other possible scenarios, but you will find in time that these scenarios are just a drain on your energies and your mental state. She has had the best possible news at this stage so you should plan for a long and healthy life.
My recommendation would be to help her to remain active, eat a balanced and healthy diet and to enjoy life.