How many weeks work for a full class 1 year?

Hi team, I have about 75% of a full set of class 1 contributions, mostly from full time work and almost all from year-round work. Last tax year was only 3 months employment, and I have been unemployed since, but it appears to register as a full NI class 1 year. I would like to plan another partial year's employment as it suits me now, and one factor is getting a full year's class 1 contribution. How can I calculate how many weeks work are needed, and how does that depend on my earnings? Is there such a thing as a partial year which needs class 3 contributions for a reduced number of weeks? (I'm sorry, I did read the info around https://www.gov.uk/national-insurance/national-insurance-classes but it wasn't enough for me to understand this aspect) Thanks, SL

Hi Spag Locks,
Please see the below link about National Insurance thresholds on GOV.UK.
This details how much is required to be earned each time you are paid either weekly, monthly or yearly.
The LEL (Lower Earnings Limit) needs to be reached for Class 1 National Insurance while being employed.
Rates and allowances: National Insurance contributions
For periods of un-employment, if you receive any benefits, they may attract National Insurance credits per week, that can complete a tax year.
If you are not reaching the LEL through employment and any credits from benefits that do not cover the weeks when you are not working, there is the option to pay Class 3 voluntary National Insurance.
We can only advise if there are any gaps within a tax year once it has been completed and processed.
Voluntary National Insurance
Thak you. 

Thank you for your reply, but I'm not sure I can understand it... sorry. The second link goes to a Microsoft login, which I think might be incorrect? Anyway I can't get any info from it. The first link suggests that by employment paying less well than my previous job, I might earn part of a year. Would I then pay a part-year voluntary contribution to top that up? Or does class 4 only come in whole years? Thanks, SL

Hi Spag Locks
Here is another link to the Voluntary National Insurance contributions information - Voluntary National Insurance: Overview - GOV.UK (www.gov.uk)
You will pay class one National Insurance on a weekly or monthly basis. If you earn less than the weekly/monthly Lower Earnings Limit (LEL), these earnings will not attract National Insurance contributions. If you earn above the LEL you will pay National Insurance contributions and these will count towards the tax year in which you pay them. 
You would not know until the end of the tax year, whether you have paid enough to obtain a full qualifying year for pension and benefit purposes. If you don’t achieve a full year, you will have the option to pay voluntary National Insurance contributions at the class 3 rate (see the link above). You would only pay for the number of weeks required, whole years would only be required if you hadn’t earned above the LEL for any weeks/months within the tax year. Please note class 4 is different and relates to a self-employed person.
Thank you.

That is really helpful, thank you. it is exactly the extra info I needed. Please would you consider adding your clarifying text from the 26 July reply to that page?