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("")))))