How long should 301 be left in place?

I have a site that has about 150 redirect 301 links hardcoded on the .htaccess file

My question is - how long should I leave them on the file, or is it good hygiene to remove them after, say, one year - or - do you just let them stay there forever?

I guess it depends if having 301's (so many) slows down the serving of code in anyway - if the answer is yes then I should probably remove...

I guess I could also check if the 301'd pages have been indexed but that is a pain and doing that x150 times is ridicolous