Hello Ideally you should use glasses all the time As your age is 19yrs,so power will not increase much now as power becomes stable at age of 18-21 yrs

Yes. All the times.

Constant usage of glasses will not change your Refractive error (glasses number), at the same time non usage of glasses won't increase your number. But it's advisable to use your glasses as far as possible, especially while doing any important work (reading, driving, watching TV/ computer etc) to have a better focus. Not using glasses in between won't harm.