Suppose : If One Campaign was remained live for 10 days and have 1% conversion rate Second Campaign was remained live for a month and have 2% conversion rate. My question is, if we say that 2 - 3 % conversion rate is excellent then Does "Time" matter in conversion rate ? How we manage time element while calculating the Conversion Rate ? What should be the minimum time required to run the campaign to find out the conversion rate ? Because obviously campaigns having more time would be having more conversion rate than those having less time would be having less conversion rate.
What you are looking for is to have a statistically significant result. This is not about "time", but more about the number of samples you have. If you can achieve having 10,000 visitors the very first day, you can collect statistically significant data within one day. If you have 1 visitor per day, it can take you 6 months. Once you have a sample data large enough, you can calculate the conversion rate and make further calculations and implications.
Within 1st 1month you will get clear idea about your conversion rate and which ads are doing fine on Google. Use Exact Match Keywords Option and you will get better overview of the exact conversion rate. Moreover, running Adwords campaign with higher Budget always results in lower conversion rates. This is my experience from past 6 years.
Of course you need time to make conclusions about your strategy and conversion rate. So analyze your statistics and make a choise now.