One of the major requirements to complete while you are working on your either your instrument rating or your commercial pilot license is fulfilling your 50 hours of required PIC cross country time. If you are fresh out of your private pilot checkride and are planning on continuing through with your instrument and ultimately commercial license, it could be very well worth your time to plan ahead on how you will complete the required flight time. Let's take a look at what the requirements are for both the instrument rating and commercial license.
50 hours PIC cross country time required for either the instrument rating or commercial pilot license:
• 1 IFR cross country flight including a distance of atleast 250 nm and 3 different instrument approaches at three different airports.
Commercial Pilot License
• 1 cross country not less than 300NM total distance with 3 landings at three points, 1 of which is a straight line distance of 250 NM. (solo)
• Complete a 2 hour, 100 nm straight line distance, dual day cross country. (dual)
• Complete a 2 hour, 100 nm straight line distance, dual night cross country. (dual)
To meet the above requirements, usually about 13-17 hours of cross country flight time is necessary. I would recommend taking that into consideration early. If you are on a budget and set timeline, meeting those requirements while you are working on accumulating your 50 hours of PIC cross country time will help you out. Most people progress from private into instrument and on to commercial. If you have met your cross country requirements for commercial by the time you finish your instrument rating, (there is nothing say that you can't) you will be well on your way for your commercial training.
There are many ways to meet your requirements and this may not suit your particular situation, but for many it is an efficient method.
Instrument Rating Flight Requirements Summary - PDF Version
Commercial Pilot License Flight Requirements Summary - PDF Version
Part 61 Instrument Requirements Federal Aviation Regulations
FAR Part 61 Subpart F - Commercial Pilots