Hi,
You'd have to know the input resistance which is due to the transformer and line resistance, but there's also some inductance to consider from the transformer so it's not too simple to figure out unless you know these things. It's simpler to measure with a scope and make a note.
There's also the forward resistance of the diodes used for rectification which may be a little higher than normal for a short time, and also the charging of the main filter cap which slows the whole process down. So there's quite a few unknowns usually.
It's going to charge much faster than the main filter caps however so you can most likely ignore this extra charge time.