There is no such thing as standard, really. Most people have + 1.00 or + 1.25 for their first Reading glasses at 40 - 45 years old.Then by late 40's have + 1.75 , then + 2.00 by 50.....then + 2.25 at 52-53 , then + 2.50 over 55. Of course, not everyone has both eyes the same power , so ready made readers aren't always comfortable to look through for more than a few minutes to read. A good 70% of people also have some astigmatism that should be corrected at the same time to make reading more comfortable. That part can only be determined by a real eye exam.
As we all know that reading eyeglasses always have generic strength typically from +1.00 to +4.00. There is always a simple eye chart tool that will help you determine the proper strength for your reading eyeglasses just by yourself without going out to your eye doctor. You can search and find out that tool online.
For most people new to needing readers, it's highly unlikely that they'll need a high powered lens. Presbyopia like many eye diseases typically starts as a minor vision impediment, and slowly increases as you age. While it's possible for people to need reading glasses that use a strength of 3.5 - 4.0 diopters, glasses like theses are relatively rare, and most people only need low powered readers. Reading glasses are rarely manufactured for strengths greater than 4, and typically range from strengths of 1-3. While reading glasses generally don't need to be as accurate as normal prescription glasses, finding the right size for you is important, as wearing the wrong size can cause headaches, dizziness, or nausea.