how long can the battery in my new mobile stay idle till first use I bought a Samsung Galaxy 6 months ago. Have not used it yet nor have charged the battery atleast once. I am worried about the battery. Can i keep it idle two more months.
You might want to check How frequently is it okay to let the battery discharge fully or as low as possible? and A: Although I was told to charge my Android device before first use, it was already charged. Ha… for this, which include corresponding hints. Some basic stuff here, though:
Wikipedia holds a detailed article on Lithium-ion batteries. For your question, the answer is mentioned there as follows:

Li+ batteries have a self-discharge rate of approximately 5–10% per month, compared to over 30% per month in common nickel metal hydride batteries, approximately 1.25% per month for Low Self-Discharge NiMH batteries and 10% per month in nickel-cadmium batteries. They last longer if not deeply discharged (depleted) before recharging. The smaller the depth of discharge, the longer the battery will last.
Batteries may last longer if not stored fully discharged. As the battery self-discharges over time, its voltage gradually reduces. When depleted below the low-voltage threshold of the protection circuit (2.4 to 2.9 V/cell, depending on chemistry) it will be disabled and cannot be further discharged further until recharged. It is recommended to store batteries at 40% charge level.

(emphasis mine)
From that you can deduce: If the manufacturer behaved responsibly (which we may assume), the battery was charged at about 40% when leaving the factory. Assuming it took at least a month to get to you (it was at store as well, so it should have taken that long or longer), and consider the fact you bought it 6 month ago, the battery wasn't charged for at least 7 months. Self-discharge rate, according to the article, is 5-10% per month -- so your battery is pretty empty now, and most likely already about to enter the &quot;dangerous level&quot; of deep-discharge.
Conclusion: You should at least check it twice a year (once every 6 month) and make sure to charge it up to ~40..50% when not using it.