Depending on how often you can fly, you can achieve the certificate in less than 1 year, flying 1-2 times a week. It can be done faster if you can fly more often. 

Most pilots should plan on 16-18 months to allow time for scheduling tests and checkrides. 

40 Hours Logged Flight Time Including

• 20 Dual 
• 20 Hours Solo
• 5 Hours Solo Cross Country
• 3 Hours Dual Instrument Training
• 3 Hours Night Flight
• 3 Hours Instrument Training
• 3 Hours Test Prep Training

3rd Class Medical Certificate

Pass Written Test

Pass Practical Test

Hold at least a current private pilot certificate or be concurrently applying for a private pilot certificate

Aviation Experience:

• At least 50 hours of cross-country flight time as pilot in command. 
• Hold Current Private Pilot's Certificate

A total of 40 hours of actual or simulated instrument time:

• At least 15 hours of instrument flight training from an authorized instructor.
• A distance of at least 250 nm along airways or ATC-directed routing.
• An instrument training using applicable instrument approaches.
• At least 3 hours of instrument training that is appropriate to the instrument rating sought from an authorized instructor in preparation for the checkride within two calendar months before the examination date.

To obtain a commercial certificate in an airplane under FAR Part 61 rules a pilot must have:

Aviation Experience:

250 hours of flight time

• 100 hours of which must be in powered aircraft, and 50 must be in airplanes.
• 100 hours of pilot-in-command time, 50 of which must be in airplanes.
• 50 hours of cross-country time, 10 of which must be in an airplane.
• 10 hours of solo practice.

20 hours of training:

• 10 hours of instrument
• 10 hours in a retractable gear aircraft
• 2+ hours cross-country and practical test preparation.
• Proficiency in commercial manuvers.