How is it that work credits expire? And how long does it take before they do start to expire?

The SSA program which concerns “work credits” is Disability Insurance Benefits (DIB or Title II).  To be eligible, an applicant (who is the wage earner) needs “insured” status which depends on the quarters of coverage that have been acquired as a result of gainful, “covered“ employment [FICA taxes].  A “quarter of coverage” is based on 1 of 4 calendar year quarters, i.e. the 3-month time period that ends on March 31, June 30, September 30, or December 31. 

A wage earner must have sufficient quarters of coverage to be “fully insured,” and therefore, entitled to DIB.  The maximum quarters of coverage needed is 40, but 6 quarters of coverage are required at a minimum. In addition to being fully insured, the wage earner must also be “currently insured.” This is the part of your question about “expiring credits.” For the period of 40 quarters immediately preceding disability, the wage earner must have a least 20 quarters of coverage in that 40 quarter period. This is the “20/40 rule.” 

In other words, the wage earner must have worked steadily and fairly recently until disability. Insured status will lapse if she or he stops working, or becomes disabled, and fails to earn quarters of coverage for 20 quarters (about 5 years).  So, if the wage earner did not work steadily during the 40 quarter periods, the insured status lapses sooner than 5 years. The wage earner must be fully insured at the time of becoming eligible in the first full month of becoming disabled. Lapse in insured status means that the wage earner is not eligible for disability insurance benefits. 

The date that the insured status lapsed becomes critical for establishing that disability began before that lapsed date in order to be eligible for DIB. The SSA field office can compute the wage earner’s date last insured, also known as “DLI”. Note: quarters of coverage are computed differently for younger wage earners.  Read more at https://www.ssa.gov/oact/progdata/insured.html