Homebrew PCBs

Hi,
Been trying to do some toner transfer with mixed success. Has anyone 
noticed any difference in success depending on the time between copy 
and ironing onto the board? i.e Is it better to do the copy / print and 
then iron straight away, or can it be delayed?

I usually make the copy / print at work on coated paper and then do the 
ironing home at night, or even the next night if I run out of time. 
Have read through a lot of descriptions, but never found any detail on 
this.

Thanks
Gerard