E30Legend, with organic models, I would expect to be sitting around 5-6K and with environment assets, (obviously depending on complexity) you should be well below that.
Don't think, 'What's my limit'. Its always good practice to keep all of your models as low poly as possible. You need to think of the bigger picture. If you are creating a huge map that will be populated with a large number of assets, you will need to keep the overall number of polys down.
When you have finished modelling, before you UV map, take a step back and look at your model. then ask yourself, what would happen if I removed that loop cut? would it destroy the overall silhouette I am trying to achieve? If not, would it ruin the look of the model? If not, remove it. on most occasions, you will see that you can actually save yourself a LOT of ploys just removing unnecessary loops cuts.
As you progress with modelling, you will build this practice into your modelling process and own't need to actually 'take a step back'.
I have seen your WIP modelling thread. You are definitely on the right track to becoming a great 3D artist. Keep up the good work and you will go far.
@TattieBoJangle, 20k on a building? Really? Is it a really intricate building? Or is it a background environment asset?