Anytime.
Protien skimmers are one of those pieces of equipment that you may not absolutely need but if your going to get one getting the best you possibly can is never a bad idea. you can make just about any hunk work but some aren't worth the trouble for what you get out of them. I've even built my own out of soda bottles and lime wood air-blocks before, they worked not spectacularly but they did work. I have had one that were so impossible to tune in accurately that I gave up after a week.