user.rb

  def time_valid?(time = Time.current)
    return true unless self.notify_on_time 
    
    Time.current >= 8 && Time.current <= 18
  end

user_spec.rb

decribe '#time_valid?' do
    let(:user) { create(:user) }
    let(:user_with_time) { create(:user, :notify_on_time) }

    it 'Time is valid for user, that can get notiies on all time' do
      expect(user.time_valid?).to be true
    end

    it 'Time is valid for user_with_time, that can get notiies only through 8-18' do
      expect(user_with_time.time_valid?(8)).to be true
      expect(user_with_time.time_valid?(12)).to be true
      expect(user_with_time.time_valid?(18)).to be true
    end

    it 'Time is not valid for user_with_time, that can get notiies only through 8-18' do
      expect(user_with_time.time_valid?(7)).to be false
      expect(user_with_time.time_valid?(20)).to be false
    end    
  end