See, I gonna make a pl map, intending it to be about as big as Upward. I'm gonna work on this map completely alone, just wondering how long it'll take?
Alright, I also worked on a payload map, and I will say this much: the way I worked on it was not very efficient. I made it one point at a time, including minor detailing, and boy it was a lot. It took me about 1-2 months of work to get it into this stage, and I'm yet to add health packs, the objective track (props are there already), spawn rooms, and functional doors (though those you can find in prefabs). This map is also fairly small as well, so if you are going for about upward size, I'm going to give some advice to keep your worries minimal from my perspective.
1) DON'T art pass during alphas
As tempting as it is in order to make your map look good as soon as possible, delay this until beta stage. Try to figure out gameplay issues first before detailing your map with various textures, props, and other crazy things. That being said, don't not
detail your map at all, and here's what I mean. Adding props that are actually important (capture point, boulders, stairs, etc) and adding both light and light_environment will help with making your map still feel like a real location.
2) Run your map frequently
Another issue I ran into while making my map was not playing on it enough. By running your map in game, you are able to see from the player's view just how broken some things are. Maybe it's an engineer building spot that is impossible to push, maybe it's a sniper sightline that peaks from one spawn to the other; whatever it is, you need to look out for these things. I waited until I had finished Blue spawn to B point to play test, and I realized just how cramped it felt. If I had to say how often, I'd say at least once for each day you work on your map.
3) Draw out your map
This is more just what I do when making a map, but I like to draw it out on paper first incase I forget the layout. This also allows you to work out your map outside of hammer, so you can see things such as sniper sight lines and layout design that may or may not be as visible in the hammer editor. Plus, if you make a mistake, you can just erase it (unless you are drawing in pen for some reason) instead of having to completely redraw the brush from scratch in the editor's 3D environment. This may effect your scaling a little, but if you used the last step, you shouldn't have to worry about scaling as much.
These are just 3 of my personal tips for making a map in general; if you don't agree with them, then...well...alright. I mean, you don't exactly have to follow my advice to the tee, but this is just me trying to help a little.