Trainer Time vs. Road Time

I personally don't like the 'drainer' all that much hence when I ride on it I prefer to do specific training in shorter time (1:30 hrs max). If I need to ride 2 hrs on the road I would then just ride 1-1:30 hrs on the trainer at a higher intensity. Make my session more specific for the given time I spend on it.

It is all about the type of session you have scheduled and on what training phase you are.

Like Jorge said, it depends on what you do on the trainer, if you are getting in a good hard workout, the general rule of thumb I have heard is that time on the trainer is equal to time and half on the road. . So let's say you ride for 60 minutes on the trainer, that is equal to about an hour and half on the road, the reason is that you cannot "stop pedaling" on a trainer on a downhill.  Although if someone just gets on the trainer and just spins very easy for an hour, that will not do anything for him/her, it needs to be a specific workout.  I have found that Spinervals or Chris Charmichael Train Right series is good to do when on the trainer. 

 Longest ride I have done on a trainer was to a Spinerval and it was 3 hours long. . it was a good hard workout. 

HR training is just a way to gauge effort (although not a direct one hence not the best). People can train using a HR, RPE, power, etc. training zones are just subjective ways for us to sort of gauge when we are going easy, moderate or hard. These zones mean little to our bodies, however through appropriate training and intensity we can ball park rather accurate how hard we have to push to induced specific training adaptations.

IMO riding the trainer doesn't translate to 1.5 of road riding. As long as the intensity at which I am training remains constant 1 hr on te trainer is the same as 1 hr on the road. Those using a power meter can see this rather easy, if you push 'x' watts the strain imposed on your body will be the same. Of course on the trainer you don't get breaks and menatally seems more challenging but riding at the same effort for the same time results on a very similar stress score. that been said even if you train on the trainer all winter and you get out as the weather improves you might need a few rides to get used to the variability of terrain which in might cause you to fatigue a bit faster. Riding a computrainer might help make te transition a bit easier.

A treadmill OTOH is a bit different depending on the model and settings but usually is easier to run on it because the belt allows you to push less (work) than on the road. You can adjust this by increasing the incline. Both (trainer and treadmill) are great tools to get training done when weather or time is a limiter, however there is nothing better than specific training (ride/run outside). Specificty, Specifity, Specifity!