Valves – doesn’t really matter. The only ones i’ve seen break are fancy (Peatys) Aluminium ones. I’ve got a mix of Stans, Mavic and DT that came with different things. Buy a load of cheap valve cores from eBay and change them when they get clogged up or bent.
How long it lasts? Depends on tyres, weather and probably humidity/temperature – our bikes are kept in the (modern, warm, dry) house rather than a garage which i think probably shortens sealant life considerably.
Does also depend on sealant – I used to reckon on a few months with Stans (about 3). I bought a big 5L container of Hutchinson and that seems to dry up in about 3 weeks. You don’t *need* to scrape old sealant out but if you just keep bunging more and more it you are adding a chunk of weight (and you can cause the tyre to stick together when you let the air out and be impossible to remove – i had to snip the bead on the last one i took off as I couldn’t get the tyre levers under it and then it was so stuck to the rim tape it lifted that as well.
How to put it in? I’ve got a couple of small sealant bottles (that came with a wheelset) – take the valve core out and use the small bottle to squirt it in. top up small bottle from large. Clean up the valve before putting the core back in.