Technique can be improved and proper workouts are easy to design, but unless you're getting in the water regularly, you will not improve. It's impossible to get better at anything if you don't practice.
If you're looking to better your swimming ability, one or two times a week won't cut it. Three times a week is sufficient, but you'll really start to see gains when you're in the water four or five times each week.
It's not necessarily important to swim for 75 minutes to 90 minutes each time. Although more time and yardage in the pool will certainly lead to results.
It's important that swimming doesn't become a dreaded chore. If you only have 20 or 30 minutes, that's fine, but 45 minutes to 60 minutes is perfect.
And really, any time is better than nothing. If you have a little extra time, go for longer, but don't overdo it either. As long as you are getting in the water frequently and you don't go several days between each swim, you will be on the right track. It's all about muscle memory, repetition and comfort in the water.