You may be out of luck. Legend 7.0 manual states:
Delay Ring. Provides a delay before the telephone rings. The length of the delay depends on the type of button and the coverage arrangement:
— On outside line, SA (including Shared SA), and ICOM buttons programmed for Delay Ring, the delay is fixed at two rings and cannot be changed.
— On Cover buttons programmed for Delay Ring in Release 4.0 and prior systems, the systemwide Delay Ring Interval, set through system programming, provides a delay of one to six rings with a factory setting of 2.
In Release 4.1 and later systems, system managers customize
coverage delays on an extension-by-extension basis. The Coverage
Delay Interval and Delay Ring Interval systemwide settings are replaced by these extension timers: Primary Cover Ring Delay and Secondary Cover Ring Delay (range 1–6 rings, factory setting 2 rings).
— In Release 4.1 and later systems, the Group Coverage Ring Delay (range 1–9 rings, factory setting 3 rings) is set for each sender’s extension. When a sender has both Individual and Group Coverage, the Primary Cover Ring Delay controls the interaction between the two types of coverage.
So, for normal line buttons, no change is possible.