Can anyone tell me how long you can keep phosphate syrup? I do not find anything about stability anywhere.

To begin with you do not indicate which phosphate syrup you mean. There is the Cafosfolactaatsyrup PB V and the cachlorophosphate syrup PB IV. The latter is very acidic so I would opt for the first one, which is still slightly more recent as the second. In the composition of this syrup occurs mainly undiluted sugar syrup. Taking this into account as well as the acidity of this syrup I think that this syrup will not really be subjected to microbiological decay and therefore long (6 months) can be retained provided that no precipitation occurs.