I read somewhere a recommendation of starting with 4 miles, and increasing by 1 mile/ ride but no more than 5 miles/ week. Also keep your speed at 10 mph or less (but I think that depends on what type of bike you are riding). The rationale for the slow speed was because if you do 2 miles out and 2 miles back, if you start off too fast you'll really be suffering on the return trip.
Those numbers are not based on much of anything at all and some people will do fine with a more aggressive plan and others might need something more gradual.
With running, I've heard the rule "no more than 10% increase in weekly distance every 6 weeks". I think other sports that are not as injury-prone as running, you can ramp up faster than that.
If you're biking 8 miles a day, I think you will build fitness rapidly. When I started biking it was just for transportation, and then later on I got interested in doing longer rides, but I had this fantastic base I'd built up just by biking 6 miles a day. I learned that a tiny amount every day is very significant.
2009 Trek 7.2FX WSD, brooks Champion Flyer S, commuter bike