What is the maximum length for telephone cable between Panasonic PBX and normal SLT phone

what i mean is the extension cable from the pbx and the phone , not the cable coming from the central office

and what is the best PBX to give me the longest lenght ?

Depends on PBX, usually 2500 ft with 22 gauge wire.
You can get extenders to go farther.

According to the TDE manual an SLT (analog 2500 set) can go 1128 meters / 3700 feet with CAT5 or 1798 meters / 5900 feet on 22 gauge.

Extender ??? can you please give more informetion about that

Any PBX manual will give max length and wire guage.

Need to go really far, use networked systems. Put in a system at far end with slt phones

Extenders:
https://www.vikingelectronics.com/product-details.php?pid=290