GPR boreholes usually extend deeper than their horizontal spacing - a rule of thumb is that boreholes require a vertical:horizontal ratio of at least 1.5 for good resolution. Of course, this guideline only applies to 2D/3D tomographic measurements (i.e., MOG) and not to 1D profiling (ZOP).
In addition, some general guidelines to the geometry of the boreholes: Minimum distance - boreholes should be horizontally spaced at least as far apart as the wavelength of the central frequency. For 250 MHz, that's 1.2 m.
Maximum distance - boreholes should also be close enough to guarantee sufficient signal at the receiver borehole. You need some information (or at least a guess) about the electrical properties of the environment you're working in to estimate this. Non-magnetic, low electrical conductivity environments are generally conducive to GPR and would allow for widely spaced boreholes (several meters or more). Forget GPR for highly magnetic or conductive environments (e.g., really salty or high clay content).