Trust me, you can always make time if you really want something. A trick I learned is to treat vocal exercises like you would an appointment. Sit with your calendar and make an appointment with yourself for a 1/2 hour. Do this for 4 days out of the week. This is strictly exercises only. Work on your songs any other time you feel like it. Make sure you always do a little warm-up before you work on your songs just to keep your voice happy and healthy.
You will find that if you stick to this routine your voice will improve much faster and you will start practicing even more because you love the results.
So, if you always wanted a better voice, give yourself an opportunity to see what can happen with practice! I dare you 🙂