My only real advice would be go early and have an extra beer or two if you’re through quickly.
I think this is what it boils down to. Flaperon has given you a reasonable benchmark. If everything is running normally then 90 mins should be fine. From the platform at T5 to the departures area in the main terminal (after security) it can be done in 10-15 mins with a fair wind and short queues. Even with a bit of queuing it should take you less than 30-45 mins.
At the opposite end, there’s a thing at T5 called “conformance” – you have to scan your boarding pass (on the way into the security area) at minimum of 35 mins before your flight departure time, otherwise you’ll be refused access on the basis that you’re “too late”. If there are security delays/problems, you may have to queue to get to the conformance check. If you hit conformance there’s no guarantee you’ll make your flight, but it’s the first hurdle you have to cross!
So, if you arrive at 90 mins before your flight, you’ve got 55 mins to get from the station platform to departures to the conformance check. In >90% (probably >95% or >99%) of cases that should be more than enough time.
The time that you actually plan to arrive then comes down to your appetite for risk, both at the aiport and with the train… and how stressed you get if things go awry.