I want to increase my vocal range. I have some questions.?

I heard scales are good for improving vocal range. Should the scale be done in whole notes or quarter notes? If I strain even alittle on a note should I sing one note lower and then work on that ? Or is it ok if you strain alittle? Very little. Say I practice 30minutes a day on my scales then how long do you think it would take for me to reach tenor from a high baritone? I'm not gonna take vocal lessons though since it cost soo much