There are multiple version of nadi shodhana described--which one are you using? And for what purpose? Generally, one starts with an equal and comfortable count for inhale/exhale with no retention and this will vary by person depending on how fast one counts and what one's lung capacity is. Retention can be added later with continued practice.
In my experience with pranayama (in 2011 I spent over 6 months dedicating 6+ hours per day to formal practice and pranayama was a big part of it), you basically have to treat the practice like a scientific experiment and very carefully observe the effects which seem to be quite variable depending on the individual practitioner and LOTS of variables such as diet, sleep, exercise, POI, etc. The effects are quite powerful which can be both a blessing and a curse--you can get results fast, but you can also wipe out hard if you're not careful which is why most texts on the subject come with a lot of warnings.
My general approach for practices that raise energy, which pranayama can be quite good for: know what your intention is for the practice and choose what seems most appropriate, then start with a very low dose and titrate upwards slowly because there can be a cumulative effect over time. For example, you could start with 3-5 rounds of nadi shodhana daily, paying very close attention to what happens both immediately afterwards and throughout the day. After at least 3-5 days of that, if no problems arise, try doubling the number of rounds. If problems arise, troubleshoot and adjust. If you can't see clearly what the effects are, continue with the same number of rounds for awhile longer until you do. After a week or two, if all is going well, you can try starting adding retentions--this is where things start to get interesting.
Capacity for energy will increase over time, but mindfulness has to keep up so it can be bad to have big energy spikes. Also, you may find that you have to adjust other variables in your life such as caffeine intake, sleep, etc. I found it to be an excellent "warm up" for samatha-vipassana practice.
This is just my personal experience. I read a number of books on the subject and found that moderately helpful, but like anything else it's about practice and trial and error was how I really learned. Best wishes for your experiments!
