Tween (full name Tween 20 or Polysorbate 20) is a common detergent used in biology used in cell lysis and membrane protein solubilisation among other uses.
- 2 years shelf life according to this chemicals manufacturer 
- 2 years for 20x Tris-buffered saline/Tween20 according to this data sheet 
Based on the SIGMA Tween20 data sheet 
- aqueous solutions of polysorbates undergo autoxidation during storage, with changes being catalyzed by light, increased temperature, and copper sulfate
- autoclaving is not recommended without testing for changes in properties; TWEEN 20 may not be stable to autoclaving, particularly with metal cations in buffer solutions.
- TWEEN 20 is heat sensitive and will darken when exposed to elevated temperatures
- polysorbates have been reported to be incompatible with alkalis, heavy metal salts, phenols, and tannic acid
- polysorbates may reduce the activity of many preservatives
- no plastic incompatabilities have been observed
- "I've recently been doing a series of experiments comparing the sensitivity of various ELISA protocols. One thing I have noticed, which is slightly annoying, is that on opening a new tin of skimmed milk (Marvel) and a new bottle of tween-20 (BDH) there are big reductions in the background. OK, so this may have been expected, but not to the extent that I have noticed. Both of the 'old' reagents had probably been opened less than 6 months ago."