I've checked the web and the official website, but didn't find anything related to this inquiry which means that the messages have practically unlimited lengths. You can test this by choosing a friend and copy pasting a large text multiple times to see if it stops. I've tried this with Viber, WhatsApp, Messenger, Line, etc and there was no limit as well.
There is however, a limit for audio messages that you can record through WeChat. The audio messages are 60 second long.
