A 120 pound person riding 29×3.0 tires on i50 rims on smooth trails might run 10 psi. A 220 pound person riding 27×2.2 tires on i20 rims on rough rocky trail might run 45 psi. Rider and bike weight, tire width, rim width, casing strength and thickness, trail conditions, and personal riding style all affect tire pressure. If you can tell us your weight, tire size and rim width, we can make a good guess. For example, I weigh 165 pounds, use 29×2.6 Trail tires on i35 rims, and run 17-18 psi.
As @Sunspot indicates there are numerous factors that a rider should take into account when setting up tire pressures. In my experience the most important are the following: tire width, rider weight (geared up), terrain, ride style and personal preference. (To me, rim width really only comes into play if your tire width is on either end of the outer limits of what is recommended for your rims, i.e. too narrow or too wide).
I’m ~185lbs geared up and I ride fairly aggressively. If I take my my trail bike (Maxxis DHF 29×2.3 and Maxxis Forekaster 2.35 20psi) to a technical trail I’ll typically set my tire pressures @ 19-20psi (front) and 23-25psi (rear). Very different than if I were riding my plus bike with 3.0″ tire width @ 10f/13r.
You’re basically trying to find the right balance between maximizing traction and comfort (think lower psi) while protecting your rims and decreasing rolling resistance (think higher psi). It’s typically better to start on the higher end and then reduce bit by bit until either you feel like you are “bottoming out” the tire when you hit hard (or the tire burps) or it feels a bit squishy when you corner hard.
I’m currently running DHR II’s 27.5×2.3 – 3C Maxx Terra / 3C/EXO/TR – 60 TPI (front and back) on i29 WTB Asym rims. Well, that’s what I had. While I still have that up front, I tried an experiment with my 2.3 in the back and swapped the i29 rim for an i35 Stan’s No-Tubes Baron rim. I now know that an i35 rim paired with a 2.3 tire is not ideal.
I’m in the process of upgrading my bike (2014 Kona Process 134 DL – drivetrain>2×10 to 1×12<, wheelset>probably Stan’s No Tubes Flow MK3 i29 paired with a 2.5 DHF – 3C Maxx Terra / 3C/EXO/TR – 60 TPI up front and a 2.4 DHR II – 3C Maxx Terra / 3C/EXO/TR – 60 TPI in back< and front fork-in that order). That being said, I’ll have to live with my failed rim experiment for now. I’m running 22psi up front and 24 psi on the back tire.
My weight (geared up) would be somewhere between 165-170 lbs.
I ride typical east coast trails (Connecticut). While there is some form of flow in some instances, the trails are predominantly slow rolling tech (80%) with some downhill gnar (20%) thrown in.
Most of the trails are undulating hills – constant up and down (100-200 yards either way) over the course of any particular trail and the climbs, more often than not, are as techie as the flats and downhill sections. I’m an aggressive rider – at least from a technical standpoint – I’m not getting a lot of speed on the type of trails I ride.