The concept of wasted time doesn't exist for a serious meditator. Little dead spaces during your day can be turned into profit. Every spare moment can be used for meditation. Sitting anxiously in the dentist's office, meditate on your anxiety. Feeling irritated while standing in a line at the bank, meditate on irritation. Bored, twiddling your thumbs at the bus stop, meditate on that boredom. Try to stay alert and aware throughout the day. Be mindful of exactly what is taking place right now, even if it is tedious drudgery. Take advantage of the moments you are alone. Take advantage of activities that are largely mechanical. Use every spare second to be mindful. Use all the moments you can.
Your practice must be made to apply to your everyday living situations. That is your laboratory. It provides the trials and challenges you need to make your practice deep and geniune. It's the fire that purifies your practice of deception and error, the acid test that shows when you are getting somewhere and when you are fooling yourself. If your meditation isn't helping you to cope with everyday conflicts and struggles, then it is shallow. If your day-to-day emotional reactions are not becoming clearer and easier to manage, then you are wasting your time. And you never know how you are doing until you actually make that test.
The practice of mindfulness is supposed to be a universal practice. You don't it sometimes and drop it the rest of the time. You do it all the time. Meditation that is successful only when you are withdrawn in some soundproof ivory tower is still undeveloped. Insight meditation is the practice of moment-to-moment mindfulness. The meditator learns to pay bare attention to birth, growth, and decay of all phenomena of the mind. She turns from none of it and lets none of it escape. This includes thoughts and emotions, activities and desires, the whole show. She watches it all and watches it continuously. It matters not whether it is lovely or horrid, beautiful or shameful. She sees the way it is and the way it changes. No aspect of experience is excluded or avoided.