Ask HN: Best project length for learning?

Hello, I am looking for a new (consulting?) job where I can gather as much expereince in different comapnies and fields as possible. I also want my work there to be usefull. I am currently working at a company that develops an erp system and I have no experience in consulting or contracting. Before I look for specific companies I could join I would like to ask you, if you can tell me a project length you found good for yourself and why.

Thanks a lot!