I think it's getting ready to go at about 5am. Stays open till about 5pm but you're much better off getting there in the mornings for freshness, variety of food and open stalls.
I'm not a great shopper but I don't mind going there sometimes. If you're into markets, then you should visit.
There are also a number of other fresh produce markets a short distance from the centre of town that open in the late afternoons. They are where my wife shops as she reckons the main market is too expensive but obviously the latter is more convenient, location wise.
Chatchai Market
Re: Chatchai Market
No, they don't sell dog meat there. I have heard that some folk in Thailand eat dog meat, but to be honest, in the 20 years I've been here, I've never personally witnessed it, and I can't recall ever meeting a Thai person who would even entertain the idea of eating dog meat. I think you will find that 90% of the population wouldn't put it near their mouths even if you paid them, and that says a lot.Although it often doesn't seem like it, most Thais are not very adventurous when it comes to try/eating different types of food, or foods which they are not well-accustomed to.
Anyway, if you like markets then this is one you should visit, but as has been said, the earlier you get there, the better.
Don't try to impress me with your manner of dress cos a monkey himself is a monkey no less - cold fact