I have used Sodium Thiosulfate as my only dechlorination detoxificant for over 30 years. (Calcium Thiosulfate also works, but is more expensive)
I usually buy the 5 gallon bucket of dry salts from a company like Fischer Scientific, and mix my own (a liter at a time), the dry salts last for years. Because I was a chemist/microbiologist at a water company I had experience buying from those type companies, as opposed to needing to use more expensive Pet Store versions.
6 months is probably the amount of time the liquid version is 100% effective, and may incrementally lose potency very gradually after that.
Some might say straight Sodium Thiosulfate is not as effective as a product like Prime, because the commercial products also add an ammonia detoxifying agent.
I always figured my bio-filter was sufficiently capable of handling the ammonia in the chloramine in my tap.
In a new tank or set up, with a wimpy bio-media population a commercial product may be needed.