Zero.
Seriously, deadlifting isn't inherently bad for your back just like jiu-jiitsu isn't. But if you taka a back fall or get hemmed up in a crucifix, that could be bad for your back, right?
Ego lifting, lifting with poor form, lifting with accumulated localized fatigue, etc. are all good ways to snap your shyt up.
Never increasing in weight is one of the surest ways to stay small and/or weak. There's also no magic injury-feee weight; creh I know snapped his pec up benching 2 plates, which was his warm-up.
There's prolly less than a handful of cats on here and NikeTalk combined pulling moar than the Sage but let them other folks tell it, if you go over 3 plates you'll fold in half.
Nah, heavy deads are what help me avoid back problems not get them.
I'm also not 40+ though so do what you will with this.
/rant
@Serious @Gully Bull @trap101 @Starski