There is no RCTs to support, but as unit practice in SL we are continuing co-trim life long. The plus point is that you don't have to worry about PcP outbreaks. In a pt, who is off co-trim, re-introduction in the wake of a PcP outbreak may raise SCr levels due to blockage of tubular secretion of creatinine by co-trim. This makes most patients anxious and will endup with variety of needless tests. In a country where PcP is extremely prevalent, we continue co-trim life long.
Duration of prophylaxis for PCP depends on recipient risk factors as well as the prevalence of pneumocystis in your region. The cost of undertaking a RCT may be too high for one to be undertaken, hence one has to rely on what knowledge is available.