Very long.
Ask the PBX manufacturer what the maximum distance is. They probably have special equipment if you need an extension even further away.
I didn't respond to this earlier. There is no easy answer.
Phone lines deal with some decent voltages, and can easily handle a few volts above/below spec... the question becomes one of the capability of the specific PBX being used.
Many residential VoiP services installed by one of the region's service providers simply disconnect Telus at the box outside the home, and simply connect the PBX to an unused phone jack in the home... This requires the PBX to deal with the run length from the jack to the box, then back from the box to the customer's handset. This can be anywhere in the neighborhood of 200 feet depending on how the electrician wired the home.
You also need to remember that any extension you plug into the jack will be an 'addition' to the wiring in the wall to the box.
You also need to consider that any non-standard wiring caused by Do-It-Yourselfers can make a huge difference if you plan to rely on any part of pre-existing wiring.
*The Box is the access box usually located on the side of your home protected by a special security screw / latch that the service personnel carry. This is usually (in a properly wired home) the single point at which ALL phone lines join together prior to being connected to the line leaving the premises.
