My fiancé has 2 boys ages 6 and 8 with a mother that is hardly ever around. The mother will go many weeks without contacting the boys. If she decides she really wants the boys to visit they have to be bribed to go to her house. Since August 15th she has not tried to even call the boys. It is now September 21st. What length of time is considered abandonment and what can be done to keep her out of the boys lives?

From six months to more than a year.

This varies from area to area so you need to check your local laws. A month will not cut it. Terminating parental rights is a very difficult task. Unless the mom presents some danger to the children its unlikely to happen.

My fiance has 2 boys ages 6 and 8 with a mother that is hardly ever around. The mother will go many weeks without contacting the boys. If she decides she really wants the boys to visit they have to be bribed to go to her house. Since August 15th she has not tried to even call the boys. It is now September 21st. What length of time is considered abandonment and what can be done to keep her out of the boys lives?
The courts will not cinsider it abandonment if the parent is aware of where the child is. I am not even family to the child I've raised for 20 months, and the parents live 500 miles away and never helped me with her finances, but child protective services told me that a mother can drop off a child wherever she wants as long as she maintains contact. So, whenever the mothers ready to take her back, there's nothing I can do about it.

There's really nothing that can be done to "keep her out of the boy's lives" unless she voluntarily relinquishes her parental rights or has them taken away. I presume that your fiancé has legal custody of the boys. Is there a visitation order in place for the mother? If so, what does it stipulate? Either way, it takes a lot more than 5 weeks of no contact to consider her as having "abandoned" her boys.