Is it likely that someone would have to increase the dosage of 1000mcg of B12 daily and how would someone know if they had to increase?

Usually, 1000 mcg is more than enough. It’s not likely that you’ll need more than that.  

The only way to know for sure is to try to increase your dosage and see if you notice a difference in how you feel. If you respond better to a higher dosage, then you can keep taking that if you want.  

But in general, it’s typically not necessary to go above 1000 mcg. You’re better off just taking a lower dosage consistently over a long period of time.