@Rival Max That depends on a few things. If depends on the shading the image has, the simplicity of the design, the amount of triangles I decide to go for and the colours. For example, if I did a simple design with little triangles I could be done in roughly about an hour, however, if it is a complicated design with loads of triangles as I want more detail and quite a few colours, then it could very well take 3 hours or more as I go through each triangle manually, also shading has a big part in it, because if there is 3d shading then I can just take the colour of the image itself as each triangle then will be a different colour so they won't merge into one huge block, however, if the shading is quite flat and blocky, I will have to change the colour manually slightly so each triangle is still a different colour. So, for example, the Chikorita maybe took an hour and a half to make, but the shiny mega Sceptile was more on the 3 hours side as Chikorita has simple colours, a simple design and large triangles apposed to Sceptile's more diverse colours, more intricate design and smaller triangles. It all depends on how detailed you want to make it really. :)
@Suicune's Fire
Is this the sort of thing you meant with creating some that were like the sprites? It doesn't look too good as it was a first attempt at trying it and I still have to find a way to get the best result, whether that be increasing the size of the sprite and overlaying or maybe even drawing it myself and colouring it by picking out shades from the sprite rather than the overlay.
Spoiler: