westheimer_rule.rb

class WestheimerRule
  UNITS = %i(seconds minutes hours days weeks months years lustra decades
             centuries millennniums)

  def initialize(time:, unit:)
    @time = time
    @unit = unit
  end
  
  def estimate
    "#{2 * @time} #{next_unit}"
  end

  private
  
  def next_unit
    UNITS[next_unit_index]
  end
  
  def next_unit_index
    unit_index + 1
  end
  
  def unit_index
    UNITS.index(@unit)
  end
end

WestheimerRule.new(time: 2, unit: :weeks).estimate
# => "4 months"