In pranayama, an inhalation/exhalation breathing ratio of 1:2 is advised. So, if you inhale for 4 seconds, then exhale for 8 seconds. However, it is not necessary to start off at a 1:2 ratio. Start off at a ratio of 1:1 and then move to 1:1.5 and then 1:2.
The duration of the inhalation and exhalation should start low and be comfortable with no strain. A good place to start is 3:3 (3 seconds inhalation, and 3 seconds exhalation) or 4:4. Then move to 4:6 and then 4:8. Progress can be something like this: 4:4, 4:6, 4:8, 5:10, 6:12. 6:12 will be your target. But take your time and do not rush.