How long does It Take For Standard US Mail to get Across The Country... ???
from detroit Michigan To New York New York?
From Detroit Michigan To San Francisco California?
the envelope is about 9"x13"
does the number of stamps it has on it affect the speed?
how fast if it has 2? and 4?
Well ... let me start out by saying that the number of stamps on a package has no impact on delivery time. There might be an exception if you use 21- 2 cent stamps on a small personal sized envelope, but that's really the exception to the rule.
Secondly, there is no such thing as "standard US mail" I'm guessing you're referring to eBay, because they allow you to choose that as a shipping option. What that means is either the seller is too lazy to figure out shipping at that time, or they're saying that they will ship your package by the cheapest method available.
The cheapest method depends on what you have in the envelope. First class mail will be cheapest, but it must be light not rigid and no lumps. After that the cheapest will be either parcel post or priority mail through the USPS. It can also be media mail, but only if it's certain types of printed material. Priority is the fastest, followed by first class then parcel post. Media is by far the slowest.
What I did was looked up zip codes for the cities you asked about at USPS.com. From Detriot to NY Priority should be 2 days and first class 3 days. Parcel post and media mail would be 4 days.
Actually I'm not going to check the other distances. Go to USPS.com and check out domestic shipping rates within the US. Don't forget that if your envelope is rigid or lumpy then you'll have to opt for package rates. And if it weighs over 13 oz you can't opt for First Class.
Don't forget shipping times are estimates. It always takes longer over holidays, especially Christmas. And they are based on business days.
Another thing to consider ... if you are the one doing the mailing. If you opt for Priority mail remember you can get free Priority envelopes and boxes at your local post office. They also have flat rate envelopes and boxes that can save you a lot of money if you use them right. If you decide to ship Priority and you want to use your own packaging ... make sure you use a ton of Priority Mail stickers on the package. If you don't the mail sorters will not notice that it is Priority, and as a result it will take a lot more time to arrive at it's destination. You can order stickers - and boxes - at USPS.com for FREE. They will be mailed to you completely free too. But don't forget they are only for Priority Mailing.