It's a bit more involved then just how long the part is in the bath, so to speak.
Decorative chrome plating consists of several steps; base metal repair/prep; copper plate; nickel plate (sometimes two nickel plates, dull and bright) and chrome plating, and then polishing and finishing. The places with good repuations have standards for plating thickness and don't shortchange you lest they tarnish their reputation.
Shortchanging any of the steps will result in a poor chrome plating job. That being said, if you get a "bad" chrome job in a 3rd world country, I would bet they skimp on the materials as that is what is most costly there. In the USA or 1st world countries, it is the labor that is most costly and that is where they would skimp (in addition to materials, too!. Because of the cost of the materials, the many steps involved, and all the hand work in preparation, buffing and finishing, it is always expensive and just gets higher--no matter where you have it done.
Decorative chrome--that's what we are talking about as opposed to electro-deposited hard chrome for tooling--is a business unto itself. In the USA there are quite a few people that specialize in it and have excellent reputations, complete with long resumes of prize-winning cars. Graves Plating in Alabama is one; B&L Plating in Warren, Michigan is another, but there are many, many more. You can search this site for some other recommendations, you can do an internet search, or look at Hemmings, too. Ask for references.
To do all the chrome on a 113 properly will cost upwards of $8,000 today and I wouldn't be surprised to see a tab approaching $12,000. I would suspect that some parts are simply less costly to replace then replate. I replaced many parts rather then replate them, and this is particularly true if the part is a base-metal casting such as a door-handle.
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored