I think I said before I used 2 litre plastic bottles as then only around 12 required and I am lazy. This season (I don't brew in summer) I ran very low on stock. I got kicked out of kitchen however with loads of stock brewed in an old fridge (not turned on) and used a bulb to keep it warm with a controller and after bottling it went straight into the shed. However with low stock only in the shed for 8 weeks and it tasted rotten. What I had to do was pack the fridge with some of the bottles to allow it to condition at 20 degrees C. After conditioning at warmer temperature it was OK.
The point I am making it needs to be in the bottles or other pressure vessel for at least a week at 20 degrees C and as the temperature drops so the time required increases. One can brew in a simple bucket but it does need some control on temperature and bottling. When I was in the kitchen the central heating was not quite warm enough. I had to put body warmers around the fermentor to keep it just that little bit warmer.
In the early days I simply did not realise how important the temperature was. Plus I made errors in measuring. I used a stick on temperature strip, and they are good, but since they are not insulated from ambient air if the brew is at 20 and the room is at 16 then the strip will likely show 18 which gives one a false idea of when brewing stalls, with a temperature probe again on the side of fermentor but since no need to see problem it has a simple sponge as an insulator one gets a more accurate reading and at 17 most kits stall, at 18 they start running, and above 22 likely to get off tastes. You only really have 4 degrees to play with. Lucky for use it is about the same temperature as we like so central heating is around the temperature we want.
As summer arrives then keeping it cool enough can be a problem which is why I stop in the summer. However a brew done a little on the warm side does improve when kept.
You must remember unlike the people doing it from scratch we are using tap water, so if brewed too slow wild yeast can get a hold, those doing if from grain have boiled the water and so using a yeast which will work cooler is OK for them. But for a kit too cold means easy to get wild yeast, once we have started getting alcohol that will stop and bugs but you should be aiming at brewing for 2 to 4 weeks, less than 2 likely it has been too hot more than 4 then likely too cold. So 19 to 20 degrees C.
Most kits say one week, I have never brewed in 7 days, 10 days is the quickest I have ever brewed a brew and that was in the summer and it had an after taste. So 14 days is about the fastest one can brew.
Also the less sugar the better is tastes, but does not keep so well, I tend to use 1.5 kg per kit, but tastes better with just 1 kg and using two kits and no sugar is really good I am told, as one adds more sugar the ABV goes up, but the taste goes down. If you want it stronger better to add less water than more sugar. The amount of sugar you can get away with varies Scottish Heavy will get away with more sugar than Yorkshire Bitter.
I in hind sight and that is easy, would say the fish tank heater is likely the cheapest and easiest way to get a good brew every time. Better if fermentor is an a builders tub and that is filled with some water with the fish tank heater than putting it direct in the brew. I however use an old fridge freezer and an 8W bulb and a temperature controller to hold the brew at 19.5 degrees C.
I would say for a kit CO2 is not required all you need is one tea spoon of sugar in every 2 litre plastic bottle. I find once opened you can reseal and it will last a week. Where with a normal beer bottle once opened you have to drink the whole bottle. Also if you do bottle too early with plastic you can feel the bottle and realise error without waiting for one to go bang. And if you have bottled too early you can simply release pressure and re-seal can't do that with proper beer bottles.