First Time Trying Hair Foils

atat

I am hoping to try and colour my wife's hair using the Hair Foils technique.  I've done it using the cap over the lockdown, which has been successful on the whole.  My question is regarding the development time.  When using the cap the colour can be applied very quickly and at the same time as all the hair is there ready.  Using the foils will take a lot longer and the colour is on from the first foil to the last foil.  If the colour should be on the hair for 30-35 minutes how does this work?  

silvercar

Do the most important foils first, generally the front. Time from these.