How long to nurture someone?

Depends on your goal and if you are properly keeping track. Is it # of touches or # of engagements (clicks, form fills)?

I suspect to get to MQL from certain form responses would take 3-10 engagements.

Nurture touch points are usually decided based on the target audience type. If the leads/prospects are highly responsive during previous engagements then a 3 step nurture is recommended but if the leads/prospects are cold then maybe a 7 to 12 touch point is recommended.

Agreed with above, but I would add that it also depends on the source of the leads. If the leads are from a high quality source you can hold on to it, and nurture it for a longer period of time than if you either are not sure about the source or don't fully trust it. With a low quality source there is always a possibility of stepping into a spam trap and therefore having regular database cleanups becomes that much more necessary.

The equation is probably something like:

[Time to Nurture] = [Length of Deal Lifecycle] + [Type of Sale] + [High Quality Lead Source] + [Level of Engagement] + [Sales Aggressiveness] - [Low Quality Lead Source]

where:

[Length of Deal Lifecycle] = do you have a short or long deal lifecycle?

[Type of Sale] = is your product easy to understand or do you have to spend a lot of time explaining/convincing first?

[High Quality Lead Source] = opt-in web forms, booth visits, personal connections, etc, etc

[Level of Engagement] = are the leads engaging (opens/clicks/visits) with your content or are they totally dark?

[Sales Aggressiveness] = would you rather maximize your efficiency (increase conversion rates) or maximize your effectiveness (low conversion rates, but more sales)?

[Low Quality Lead Source] = list purchases, etc

What else can you guys think of?

As a general rule of thumb I say "if you want to be conservative, give-up communicating to non-engaged leads after 6 months and if you want to be aggressive give it 2 years."

Thanks Grant, formula approach puts it together in a nutshell!
Some attribution modelling about the lead life cycle definetely helps in figuring this out, when drawing the lead lifecycle you could add there all the touching points that marketing is creating, also other channels than nurturing. With that customer-centric approach it will be easier to decide when an how can we provide relevant added value with nurturing.