I don't have one answer in any case! (my answer is only relevant to baptisms in the Church of England )
If I have other evidence, such as a sequence of children being baptised at regular intervals, or an age at burial etc then I'd put about with the year and occasionally, the month and the year (especially if the baptism is later in the month )
If I don't have any other indication then I would put before (and with some of my 19th Century ancestors it could be several years before . They often had their children baptised either as an emergency when the child was close to death, or baptised in batches. (quite often these batches happen a week or so after the emergency baptism)
In general earlier baptisms (ie those from the earliest registers) seem to have been quite close to the date of birth. The rubrick in the book of common prayer from 1549 says that baptism should be done publically on a Sunday or Holy day except 'in extremis' (and you will often find in the register, privately baptised in that case) The clerk was also told to oft admonyshe the people, that they differ [defer] not the Baptisme of infantes any longer then the Sondaye, or other holy daye, nexte after the chylde bee borne, onlesse upon a great and reasonable cause declared to the curate and by hym approved.
If the baptism took place just after the Commonwealth period, then the person could be an adult as many infants missed baptism during that period (the restoration church in the 1662 BCP, introduced a new service for the baptism of those 'of riper years') In this case it could be very many years after the birth