The length of stroke really is dependent on the brush you use and how you load it. The open time you have depends on the paint product and the temp/humidity of the room. So what I get is not necessarily going to be what you get. I don’t work the way other people might and the way I paint might make other experienced painters cringe/laugh/spit out their tea.
However, lets assume I am painting a bit of door trim. I load the brush. Now my method now is to load about half way up the brush but I don’t do the tap out that most decs seem to do. I wipe one side. Remember the brush is loaded with paint inside anyway. So the un-wiped side I apply working down and increasing pressure the further the stroke goes - and I am going to put about 30cm on the wood. Then I’ll either put the brush back above the point I started and spread down again to even out the paint before laying off upwards from the end of the stroke and into the previous stroke. That’s it - don’t touch that section again. Depending on the paint and the area you are painting you can even miss the second downward stroke which means you simply apply and spread downwards and then lay off with a single up stroke. The trick is knowing how heavy to put the paint on so you get maximum coverage and minimum risk of runs and drips. I know a lot will spend more time spreading and laying off but with the right paint thinned to the right consistency you’ll find the less you touch it, the better it will level off with minimal brush marks. This is something you need to work at and you will find your own method and your own brush/paint preference.
Working door frames I generally work from the top down and paint the whole frame rather than going in strips and coming back to the top. But you do need to work fast to make sure you’re always laying off into wet paint.
Best tip is to buy yourself a few lengths of trim and experiment with different brushes and methods to see what works. Don’t take what other decorators tell you as being the only and best way. Use logic. You know what you need to achieve as a result so work through how to get it. Getting it wrong is the fastest way to find out the best way.
Walls - generally the reason people use a mini is to keep the roller texture going as close to the edge as possible. That does work and it makes the brush cuts less noticable but it also adds to the work time. Some people have problems with ‘framing’ where the edge stands out against the rest when the paint has dried. I’m not convinced this is a paint problem though. I think more often it’s that we sometimes put less paint on by brush than we do with a roller. So when we’ve finished the paint is a bit thinner round the edge. So the obvious solution is to get a good coat of paint on when you cut the edges. If you’re worried about going like the clappers to get it done so you can roll while edges are wet, then you are likely to put the paint on too thin. So plan to roll the walls the next day - or at least after a few hours - and concentrate on getting a good thick coat on the cuts with the brush. BUT feather the edge where you’re going to be lapping with the roller otherwise you will see a line. If you’re working with a mini that’s less of a problem but it can still show in the final finish if you leave a heavy edge.
Brush cut, mini roller, big roller - one wall at a time? I have worked that way but it’s so messy and I don’t think very effective. While you’re doing your big roll on your brush and mini roller are sitting there going dry and vice versa.