If you are not sure about the integrity of your RNA means you didn't run it on the gel to check 28s/18s ratio so better to use good concentrations you have. The usual available kits for RNA-to-cDNA conversion in general has the capacity till 2 ug (2000 ng) of RNA, but till 5 ug also now available but are expensive. So from 1000-1500 ng of RNA would be fine. Then check your cDNA concentrations. Properly and equally calculated cDNA of all the samples of your study is very much important for your qPCR uniform results. We are usually diluting the cDNA  to 50 ng working dilution for qPCR. And as Nandini, once you made cDNA of your RNA so get relax,  that undiluted cDNA remains stable even  after a year if kept at -35 C. We never faced any problem in this. Only we make fresh dilutions from the cDNA stock when we need. Also you can try to use applied bio systems or any other RNAse Inhibitor for your cDNA conversion step (RT-step) to further boost your cDNA yield. But the main theme in working with RNA is to adopt a real careful, clean, sterile well maintained working bench and accessories to avoid loosing your RNA and in such case the first step which is the RNA extraction needs extreme caution and care to prevent degradation of your RNA during this phase. No matter you store your RNA at -80 c every time you will measure you will have a decrease yield of each sample so quick conversion to cDNA after RNA extraction is really vital.

i wont advice you to use less RNA just because your dilutions can make some transcripts not be transcribed, hence use applied high script cDNA synthesis kit to do the same  were we use 10 ul of 1 ug dna