For me he was still a quality manager between 2006-2008. His teams displayed a world class technical level and he got the best out of them. His team in that period went from fighting for top four, to being genuine challengers for the Premier League and Champions League trophies. The worrying aspect of his management in this period though were our weaknesses as a team defensively.
I think Wenger started to rapidly decline in general back in 2009. After the end of season collapse of 2010 he was finished in my eyes. We knew back then that Arsenal were never going forward under him by this point. He was only going to get worse at his job.
Arsenal should have parted company with Wenger in the summer of 2011. It was the second grand end of season collapse in a row. That was the right time for change. Instead his position didn't even come into question.
Unsurprisingly in that time since, he has progressively got worse at every aspect of his job, even what he used to do well. As a result his Arsenal seasons have been very similar since 2011 and his Arsenal sides have only performed worse and worse as the seasons roll on, to the point where by 2012-2013 we no longer had the technical level and attractive football we were renown for.
2014 comes around and Wenger finally wins a cup competition and has the chance to resign on a high note, leaving a confident squad and upbeat set of fans behind him for a new manager. Instead he signed a three year contract and all the talk was about the money Arsenal now have...
To his credit he won another FA Cup and assembled a decent squad with big money being invested into the squad, but in terms of overall performance his Arsenal side has continued to only regress as has Wenger's decision-making.
With summer 2017 approaching, we face the prospect of another two years of potentially irreparable damage with this man at the helm.
All in all, he has overstayed his welcome by at least three years and in my opinion by about six.