I have known some guys to get off the plane on their first visit and be married within a couple of days but unfortunately the vast, vast majority of these have ended in divorce or permanent separation.
If anyone has done that in the past and are still married 5 or 6 years later then I take my hat off to then as they will be quite an exception.
It is difficult to say just how long to wait, should it be on the first,second or third trip? It is hard for anyone to say other than yourself. But do not be caught up with the moment. Do not let anyone co-herse or almost blackmail you in going ahead before you are ready.
Most who come on this site are not in the first throes of youth and as such you should be able to weigh up the pro's and con's. Is she really in love with you or is it just a green card she is after. When she says 'I Love You' does she really know what that means, has she had time to show it in ways other than words? I saw a recent programme on love and marriage and many of the Chinese interviewed had no idea what 'Wo Ai Ni' really meant. Most thought it was just a phrase that their partner wanted or expected to hear!
Remember getting married is easy in China, the hard part is getting the lady back to your home country after marriage. So base your plans around that.
Willy