Swimming, not winning

Been a bit of a swimmer back in day(not like im old now though..)
Swimming is mostly technique but fitness plays a big part too. Typical example is the TI following(total immersion)

You need to fix major errors in technique first before pushing on fitness, then minor techniques will come with practice.
Get muscle memory sorted with swimming, thinking about what your hands are doing is draining mentally and WILL tire you.

I suggest this site, swimsmooth.com
Also youtube, the swim club is good.

In relation to your questions, It takes about a month to get a base fitness for swimming depending on ability, if your completely useless then x by each major flaw with your stroke lol.

But generally you can get "good" pretty quickly but to be a great swimmer or sub 1 min 100m ect an take years.

Dont be put off though swimming is great for your lungs and also rehab for injuries.
If you need any pointers im happy to help