Clay bars - once opened consume within how long?
As the title says, how long can I keep a clay bar once I've opened it? Obviously I could keep it forever but it wouldn't be much good for detailing after a while. If I wrap it up in clingfilm, could I keep it for a week or so? Reason I ask is that we've just changed our family car (bought a 2007 C4 Grand Picasso) and the paintwork needs some TLC, but I'm not realistically going to be able to complete it all in one day. Most likely I'll clay, polish & wax a panel at a time during the evenings.
Second question, how do I know when to discard the clay and replace it? I've clayed my S3 before but didn't pick up much residue (it's well looked after!). However I think the Picassso is a different story...