Lifting heavy things

How do you know it’s time to stop lifting heavy things? I work at a paint store and there are times where I have to life 5 gallon buckets. My doctor has been fine with it since I’ve been doing it for years.

Yes it is getting harder, because I can’t use my abdomen as a brace, I use my hips now (like carrying a toddler), but how do you know when to stop?