Often there IS no number of days in advance to buy tickets. Going to and from Beijing is often so packed that no tickets are ever available because many travel companies buy all the tickets before they ever hit the sale desk.

I once spent an agonizing 2 hours in line at the train station only to be told that there were no trains in a whole two week time period going to any town in all of inner Mongolia.

Save yourself the hassle and book a long distance bus. that or talk to a travel agency.

ten days but before festival or holiday, it is real hard to reserve a satisifying ticket.

Depends on the train, whether it is holiday time and where you are going. On some of the high speed lines you can get a "C" or "D' train almost any time you want booking on the day or the day before. Beijing - Shanghai, Beijing- Shenyang etc In holiday time it's a totally different story as the whole country seems like it is on the movce and all forms of transport are heavily booked out.

10 days is the written rule. Not sure how many people abide by that because I just bought one today, actually bought 11 for the group, and we aren't travelling until the 6th October. We had to argue to get 11 but eventually won. You just have to remember that in holiday periods you are wanting to do the same as 100 million other people. That's 100,000,000!!! Last Chinese New Year they estimated 140 million people wanting tickets. That makes it frustratingly impossible at times. I was stuck in this city for a week before I could get a plane ride out. trains were booked even worse. It's a steep learning curve.