Furthest I have traced back on my paternal side is mid - late 1400's on one or two lines that were yeoman farmers or gents and left good documentation such as wills and land records. Majority of lines I have had the time to trace however are back to early 1500's all documented. Was fortunate that many parish registers went back to 1530's.
On my maternal side it varies quite a bit. If you are looking at furthest lines, with documentation being parish registers, wills, land/manorial records, inquisition post mortem's, pipe rolls, etc, then it would be mid 1200's. If you include visitations, pedigree rolls, landed gentry records, etc. then further back again.