[Mosin91/69]

The stock silverback bucking is 60 degrees and made of silicone. I installed the Silverback m140 90 Newton spring and I'm shooting about 320 fps with. 40g bbs. Will I need a harder bucking or do the stock Silverback buckings typically last a long time?

 

[SiliconeSword]

The bucking should last a while, but it will wear out eventually.
I recommend that you use it until it's worn out, unless you have another gun to put it in.
In the future I'd get a 70° or an R-hop and Faceless bucking from Faceless, but you should be good for quite some time.