Hey, I was just wondering how long it takes most people to get a really nice tone on viola?
So far the technical side of songs is quite easy.. but it sounds rubbish without a good tone! Any tips for improvement, or does it just take time and practise?

time and practice. However, try to hold the fingers tighter on the note, and dont press very hard on the bow. that should help a bit

I agree, practice and time are essential for a good tone. But keep in mind, good tones come quicker if you feel the music more than read the music.
I also have some tips:
-play confidently (have a firm hold of the bow, but not too tight and play closer to the bride)
-if you can, use vibrato
-play with expression
-make sure your hands are loose but have firm control.
-to improve your C string specifically, switch the C and G string tuning pegs around so the C is on the 3rd peg back and the g is on the 1st. Depending on the viola, several players think that the extra tension on the C string helps it sound richer. My teacher uses this technique and suggested it to me. It does wonders!