It may depend upon what you are trying to simulate in your experiment: A single dose of high salinity irrigation water, or a regular supply of low salinity water, or growing plants on an already saline soil etc. How long will the experiment run for? If it is a two-week arabidopsis trial, 1 dose would probably be enough, but if it is a salt-tolerant shrub growing for 12 months, maybe several small doses of salt simulating intermittent rain events would be more appropriate.

Are you hoping to see progressively greater symptoms over the course of the experiment, or are you expecting some plants to survive and some to fail? You haven't mentioned the treatment and control groups - is it different levels of salt on the one variety, differnet soils, different salts, different concetrations of salt or different volumes of water? Each of these will influence the way you run your experiment.

It sounds like the pots are free-draining (i.e. have a hole in the bottom). If you plug that, you won't have to add as much water to the pot while retaining the salt you added in the beginning. You will have to limit the water to prevent saturating the soil, so that may be a less-optimal design if you are trying to simulate a different water/salt regime.

You should add saline water in soil until desired ph or EC is acquired.you should check soil EC or pH periodically. If it is maintained you can add more saline water

That's an excellent question. Thanks.

Weekly saline plant watering, in my opinion, is more in line with what farmers would typically do. In the Mediterranean region, water used for watering plants is usually salty. To increase yield production, farmers irrigate their plants at least once a week. So to be closer to the natural conditions, it will be better to irrigate your plants regularly with the desired salt concentration. But, to be more exact and as the other researchers already suggested above, you can also measure the EC after each irrigation.

Good luck.