There really isn't a rule of thumb for maximum length - it all depends on the particulars of the installation.
You will have two issues;
1. Final invert at the septic tank.
2. Solids settling out in the pipe.
With a long run, you will have a deeper pipe connection on the septic tank the farther you run the pipe. Septic tanks have to be pumped - and sometimes maintained - so the deeper it is - the harder it will be to maintain.
The longer your run - the more likely solids will settle out before they reach the septic tank. Having a lot of fixtures that flow water without appreciable solids (showers, mop basins, etc.) help push the solids down the line. Extra water closets discharging to the line also help.
A 2% slope will cause the solids to travel farther on less water - but will also cause the final pipe invert to be twice as deep as the standard 1% slope allowed for 4" pipe.
Code (at least the IPC) require cleanouts every 100 feet to help push the solids along if they start clogging the pipe.
I have seen runs as long as 1000 feet - but there were other fixtures discharging water into it, the final invert allowed for it, and there were a good quantity of cleanouts.
