It probably depends on how far the language is from the developer's experience. I can imagine it will take a developer only familiar with ruby, python & js qui

It probably depends on how far the language is from the developer's experience. I can imagine it will take a developer only familiar with ruby, python & js quite a while to become productive in assembly language. However, a good developer that knows e.g. python, php & c really well (moreso if he generally know *NIX really well) will probably not take an inordinate amount of time to become productive in perl.