What’s important to know is the distance at which you can't make 50% of your putts. To do so, I suggest that you try putting 10 balls at a distance of 3 feet from the exact same point. Next, continue the exercise by gradually increasing the distance by 1 foot between each series of 10 balls until you can’t achieve the 50%. This will give you the exact critical distance at which you succeed one time out of two.
During your next training session, start your putting practice with your 50% distance as mentioned above and with time, you’ll notice that it will become much easier for you to complete these putts.
For measurement purposes, I suggest you take a look at Mark Broadie's statistical analysis grid which compares PGA tour golfers putting average to an 18 and 0 handicap player.
You’ll notice that professional PGA players are incredibly consistent in their shots in regards to the average amateur player. While this is true, know that it is always possible for you to get better.
I suggest filling out your own column and see where you stand and where you can improve.
If your distance is lower than the one shown in the above chart at 18 Handicap, don’t worry! Practice makes perfect.