There is no exact weight. It depends on several factors including weight of item being lifted and are the rear tires filled, or do they already have weights bolted to them ?
Ideally you want enough rear ballast to allow the tractor to lift/carry the front 'load' safely but not way too much to cause excess strain on the tractor(or put ruts in wife's lawn !). Too much ballast will cost you in excessive fuel,extra strain on tractor parts.
The owner's manual may have a chart with suggestions,or in the 'loader operations' section ? My BX23S has never been without it's backhoe(+-700#) so haven't needed to 'dial in' a ballast.
If you're moving bucket fulls of mulch, maybe 2-300#s ? Moving 1000# bales, 500-600#s ??
Hopefully those with your series tractor will respond,pretty sure they'll ask 'are the tires loaded ?'