Assuming you're referring to a marketing email nurture campaign:
The length of your email nurture campaign will depend on a number of factors, including your goals, your audience, and the type of content you're sharing. However, a good rule of thumb is to keep your emails short and sweet.
Your emails should be focused on a single message and easy for your readers to digest. Long, drawn-out emails will only frustrate your readers and likely lead to a high unsubscribe rate.
That being said, there is no hard-and-fast rule for how long your email nurture campaign should be. Ultimately, it's up to you to experiment and find what works best for your business and your audience.