I usually give my cleaner 20 dollars on Christmas and New year and 50 dollars on our anniversary.
Part-time? I mean, not a live-in helper? Seems to me that every employee gets a "13th month" payment at Lunary New Year, so that's what I do. I have a once per week, 2 hours or so cleaner in Taiwan, was 4 hours in HK. Both places I gave one month at Lunar New Year, and around 1/2 month at Christmas (that's on me, that's my holiday...feel like I want to give something). Both very trustworthy and reliable, also paid my HK cleaner for taking care of cats when I went to the US... (which is the only reason I hired a cleaner in the first place-since no pet sitter wants to come to Peng Chau...).