Most of the societies that we collect from are able to collect royalties as far back as 2-3 years. We make no guarantees about our ability to collect retroactively, as this depends on the individual societies and may be affected by any past publishing deals you may have had in the past.
However, the best way to make sure you are able to receive retroactive royalties is to make sure you have added all of your titles to your account with the correct co-writers and splits, along with ISRC/recording information.
If we successfully collect your retroactive royalties, this payment will most likely come two or three quarters after your first royalty statement from Songtrust. For example, if your first royalty statement from Songtrust was Quarter 1, then you can expect to see retroactive money start coming in during Q2 & Q3 of the same year.