Recent experience (90-day rule)

For Part-FCL licences, FCL.060(b) says a pilot "shall not operate an aircraft in commercial air transport or to carry passengers":

(1) as PIC or co-pilot unless he/she has carried out, in the preceding 90 days, at least 3 take-offs, approaches and landings in an aircraft of the same type or class or an FFS representing that type or class. ...

(2) as PIC at night unless he/she:

(i) has carried out in the preceding 90 days at least 1 take-off, approach and landing at night as a pilot flying in an aircraft of the same type or class or an FFS representing that type or class; or

(ii) holds an IR;

UK national licences such as the NPPL have an equivalent "recent experience condition" in ANO 2016 Schedule 8 Part 1 Chapter 1 para 1, requiring the take-offs, approaches and landings "as the sole manipulator of the controls". Para 2 adds a "recent experience exception" for aeroplane and helicopter private pilot's licences: one passenger who is also qualified to act as pilot in command on the flight, who has been told that you don't meet the condition.

CAP 1590 (guidance), para 15 repeats the rule for cost sharing: "To carry passengers, a pilot must have completed, in the relevant class or type rating, three take-offs, approaches and landings in the preceding 90 days."

It is a rolling window. If your last three landings in the class were 91 days ago, fly solo or with an instructor before taking passengers. The rule applies to every passenger flight, not just cost-shared ones.