33P decay and autorads

[C.S. Wilding]

Hi,
Does anyone know whether there is a table/graph/equation available that 
gives a rough idea of how the exposure time of an autorad will increase 
with time since production of the labelled compound. I am using 33P for 
AFLP so there may be variability in band intensity, and I realise this 
will therefore only be a rough guide. Related to this, when should you 
give up using 33P as it will be just too weak to bother with.
Craig.