Using toNumber
Number must be at the start of the Frequency options.
Entries
Next Date
dateAdd(prop("Last Completed"), toNumber(prop("Frequency")), "days")
Using replaceAll
Number can be anywhere in Frequency option.
Entries
Next Date
dateAdd(prop("Last Completed"), toNumber(replaceAll(prop("Frequency"), "[^0-9.]", "")), "days")
Using Nested if
Formula would need to be updated if you added a new Frequency option.
Entries
Next Date
if(prop("Frequency") == "Weekly", dateAdd(prop("Last Completed"), 1, "weeks"), if(prop("Frequency") == "Fortnightly", dateAdd(prop("Last Completed"), 2, "weeks"), if(prop("Frequency") == "Monthly", dateAdd(prop("Last Completed"), 1, "months"), fromTimestamp(toNumber("")))))