You're not going to make a competition worth piece in 20 minutes, but that does not mean you cant use simple and fast techniques to make great looking tabletop terrain in a very short amount of time.
The Elven air spire (Part of my July Demons and Elves Patreon release) is a modular building with 4 floors and a roof (although I only did the ground floor, floor 4 and the roof in these photos). Floor 4 and the roof can be used as a free standing Gazebo separate to the spire, so you're getting double terrain for your painting time.
I only used 3 main colours, plus 4 or 5 extra colours to pick out some of the details.
Prime the model in black
Heavy dry brush using burnt umber
Moderate dry brush with clay (don't paint the roof)
Dab on extra spots of clay colour (don't paint the roof)
Light dry brush with pumpkin (don't paint the roof)
Dry brush the roof with a dark red, followed by a lighter red and a little orange
Go over any parts which are going to be bronze. I chose to paint the roof struts and the decals over the walls with burnt umber
Go over the brown lightly with a bronze colour, I used a cheap craft paint by docrafts all purpose acrylic paints called Metallic Bronze. Make sure you have only a small amount on your brush as you want the underlying brown colour to show.
Paint the flags, I chose a few purple colours starting with hexed lichen (Game colour by Vallejo) then blue violet (also by Vallejo)
Add some flocking to give a little texture and character.
Sit back and have a cup of tea enjoying your 20 minute painted Elven air spire.
You can find the Elf Gazebo Model on my store, or as a digital reward for this months Patreon.