Really depends on the female in question. Back to back breeding can be harmful to a female that does NOT do well at regaining her lost body mass after parturition.
That said, if you do have a female that can regain her mass well, she can be bred back to back for ??? years. Take my girl Boo for example. In 2001, 2002 and 2003 she gave me trash litters, mostly slugs and a few stills. 17, 25 and 25 respectively. Had her for sale as a pet only (infertile) in the fall of 2003, almost sold a couple of times but nobody came up with the cash. Left her in with the rest for that winter and the next year - 31 babies, my largest litter ever. I guess she got the memo Produce or be sold. The following two years 2005 and 2006 she produced 26 babies each year. That's a three year total of 83 babies (and 5 slugs). Quite impressive!! The six year total is 155 eggs produced or 26 eggs / year!!!
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Thanks,
Dave Colling

www.rainbows-r-us-reptiles.com
0.1 Wife (WC and still very fiesty)
0.2 kids (CBB, a big part of our selective breeding program)
LOL, to many snakes to list, last count:
24.36 BRB
19.19 BCI
And those are only the breeders 
lots.lots.lots feeder mice and rats 

