You can apply during the time you are taking your prereqs. As long as you finish your classes before matriculation into your accepted pharmacy school, then you're good. However, some think it is better to have completed at least some of your higher-level classes (Organic I, Calculus, A&P) before applying. Also, some schools allow students to take classes during the summer before admission, and other don't...so look into your desired schools to find out more info.
I have talked with pharmacy students who were able to get in only after 2 or 3 years without their undergrad degree. One of them had a 3.5 and a low 70s PCAT, which are good stats, but not extremely impressive.
There are other factors that contribute to acceptance. Pharmacy experience is a big one, but having good LORs (letters of recommendation) and taking part in extracurriculars can look really good too.
No matter what, it's good to apply just to see if you can get in. If not, then apply the next year...schools love seeing re-applicants because it shows their desire to go to that particular program.