advise needed on neem oil

ello evry1 i have purchesed myself some neem repel oil.. well on the bottle it says to use 20ml per litre of water.. that sounds ok to me.. but it allso states that you should use the mixed solution within 6hours or mixing... well there is only 250ml in the neem bottle .. and as you only need to spary bout everyweek or 2.. it seems a waste not to use the left over solution.. surely you can?? becoz if you can that means you only get 12 mixedsolutions per bottle and 12 sprays.. so the bottle wont even last a grow if you spray once a week.. hmmmmmmmmmmm.. could i use say a quarter of a litre with 5 ml of neem?? that way 1 bottle costing £19.95 will last a whole grow.. allso how old or how many nodes should my plants have b4 i spray them with neem?? thanks all..:thumbsup: