I'm one of those casual types who don't formally 'practice'. I just play, at whatever level i currently am, with the faith that if i play more, i will get better slowly.  

I find that with any activity I take on for enjoyment alone, the natural tendency is to give it the lowest priority after first attending to all the work and chores. That means that too often I'm too tired to give it importance at the end of a busy day, and I wind up skipping it. 
I've learned that I must carve out the time for practicing the things i love and schedule it right into my day, rather than as an afterthought. Even if it's only ten minutes, I must make the time to do my music or art or other hobby, giving it the same importance as I give to work or the other items on my "to do" list for each day. Doesn't really matter if I make any creative advances that day, it's often just the act of doing or 'honoring it' that benefits me. Whenever i carve out that 15 minutes or so in a normal or in a hectic day, I have never regretted it.

One of my favorite bands.  Though their "Dance Till We Die" rap garbage, complete with expletives, is a track to skip.

Ah, I like a bit of OMNIA too... 

I'm playing mostly Irish tunes with the goal of playing at a local brewery for St. Pat's 2022.   I much enjoy Medieval and Renaissance music and have put together some sets.  There are also dabble with some less than trad tunes...

OMNIA (Official) - Fee Ra Huri - YouTube

Star Trek TNG "The Inner Light" Music - YouTube

Warming up the whistle by blowing breath through it before playing does seem to help with getting the high notes.

Cuts & rolls seem to be fairly important to the way Irish music is played, so if that is your choice, it will likely help practicing them too.

Tune playing is my general approach too.   Sometimes I warm up, both my self and the whistle, by doing an ascending/descending scale or two.   Online and printed tutorials recommend exercises with cuts and rolls.  Perhaps these would benefit me but the draw to paly tunes is stronger.

Sorry, I haven't been getting notifications & only just seen this - I think it mainly depends on your end goals.

Personally, I just play a tune or two, I don't actually structure my practice time, I only play for my own pleasure, & it works for me.

Playing scales teaches where the notes are, but equally, you could just play from tab.

How do you structure effective practice time?