When did Nickelodeon go downhill for you?

Well I started growing out of it when I was like 11 so I'd say around then.

I thought it started to go downhill sometime around 1998 but didn't fall off the cliff until 2003.

I don't know if this is a good enough of an answer, but this is what I just found on Reddit:
https://www.reddit.com/r/nickelodeon/comments/5eygiy/what_really_killed_nickelodeon/

So a lot of people hate modern Nick. Why is that? Well, it's because the sitcoms and cartoons that are being made now (excluding The Loud House, that's the only GOOD 2010 cartoon.) are trash. What started this was actually Spongebob. This makes sense as Spongebob is Nick's largest running cartoon, spanning from 1999 to now. Because Spongebob is such a big cartoon, it takes lots of work to make just 1 episode. This is one of Nick's most popular cartoons, so it gets more attention. Other shows such as Breadwinners or Sanjay and Craig are crap only because the writers and animators don't give a damn about those cartoons, and just care about the money. You can obviously tell as when Nick had the decision of choosing Adventure Time or Fanboy and Chum Chum to add to the network, Nick chose Fanboy and Chum Chum! Then we have the sitcoms. Every show Dan Schneider has produced has done AMAZING. Drake and Josh, iCarly, Victorious, every show was great. Now we have Henry Danger, which a lot of people hate and like. It really seems Dan is lacking in work lately, probably because he wants shows people like, and 1 mistake will make people hate him. For the actors, a lot of the time they don't try their best. If you've seen lists for the worst sitcoms ever, you can see the actors don't even try. Disney has a lot of problems with actors, but that's beside the point. To recap: Cartoons are bad because they aren't having time spent on them, or the people working on the show don't care,

Sitcoms are bad because the actors don't try or (in Dan's case) get too distracted of the shows they've done in the past.

Hopefully you agree with me, this really is a big problem with Nick.

To me the downhill slide started around 1998 but didn't get really bad until 2003 (when it became unwatchable).

When show quality took a back seat to the almighty dollar.

The late '90s.

When they canceled Drake and Josh because the boys weren't cute anymore