I have been buying bitcoin when the price was around 400$-450$.Most of my bitcoins are bought at that price around 40 BTC and remaining at less then 600$ price.i have been holding btc in paper wallet since then and already my stash in usd is 6 times.
here is one of my tx

what i want to know is please suggest some ways to invest my bitcoin at zero risk to earn some profit.Even 0.8% to 1% profit per month is good for me but it should be risk free.i don't mind the profit percentage but risk should not be there.if you have some suggestions please post that too,if i should keep holding for more years and if yes how long should i hold?
This is the truth that the investment of the Bitcoin is a risky job but still if we can invest in better way so that our investment will not face any big risk in any case .
First of all you need to go at the poloniex and create account and then deposit your Bitcoin and after that transfer your fund in the lending , then you will be able to make easily more than 3-4% per month .
And if you want invest more then you can invest in yobit where you will get 0.2% of your investment ( invest box ) , but problem is that you can't invest more that 10btc Thier but still I think that is much better option to invest because it doesn't involve any type of risk at all .
I think these small profit are enough for you but still if you are not satisfied with this earnings than try once your lick to start a business in Bitcoin like to start casino or anything else .