I use idophor with great success. I do keep it in a spray bottle for a while. The thing with idophor is the active ingredients will evaporate so if the spray bottle is not full it can lose some effectiveness over time so I open the spray bottle up and give it a sniff before using it. If it still smells more or less like a swimming pool I figure all is well. but as others have said it's pretty cheap. I buy a big bottle for ~20.00 and it lasts for a year or more. Also it is effected by light so an opaque bottle is a good idea. that being said, at a cap full for 5 gallons I generally mix up a batch in my fermenting bucket at the begining of the brew day and transfer it from vessel to vessel as I work ( I vessel as I verk). When I am in need of a small amount I will mix up ~1/5 of a cap full to a jug of water and use that to sanitize my wine thief for samples, airlocks etc.
I also understand that eventually you will breed some bugs that are partially resistant to whatever sanitizing product you use and you will then have to switch. **knock on wood** this has not happened to me yet.