I've added a defautl duration as required for my usecase.

It changes the signature to the Parse function but by using vargs it shouldn't impact any existing usage. I have extended the test cases to match too.

A futher extension I would consider for my use-case is: DisableCustomDuration to force the default duration. (Or perhaps it would be "ForceDuration?")

I haven't found a "great" way of doing opt args for options. This is typically how I do it. I have also seen people use it as an internal struct modifier ie:

type Opts struct {
 defautlDuration *time.Duration
}

type ParserOpt interface {
  Apply(o *Opts) error
}

type DefaultDuration time.Duration

func (dd DefaultDuration) Apply(o *Opts) error {
  var t time.Duration = time.Duration(dd)
  o.defaultDuration = &t
  return nil 
}

...
for _ opt := range opts { opt.apply(&opts) }
...