In a RES bounty farming situation or similar and assuming that you have upgraded the default life support to D grade, the Oxygen lasts for 7.5 minutes, which will guarantee you enough time to get to a station that's within 1000ls of the RES. With luck, you can get 1500ls, but if the station is 3000ls away, you're in trouble. Also, in your trip to the station, you have to fly directly away from the planet with rings until you're clear of its gravity before you turn to the station., and you must avoid going close to other planets along the way. I learnt all these things the hard way. you have a chance to learn from my mistakes.
To summarise, if the station is more than 2000ls from the battle-grounds, don't go there in a ship that's likely to get its canopy blown off when you only have d grade life support.