It's Christmas soon. How much should I give our domestic helper as a Christmas bonus?

Agree with a gift. Otherwise, you could follow Timothy Tiah's suggestion where he gave his helper the choice of a cash gift OR the choice of choosing gifts for her loved ones. http://www.timothytiah.com/2015/12/21/what-my-filipino-helpers-family-got-for-christmas/

I know Christmas is over but just in case you were wondering again for Christmas next year. Haha. I would give a gift instead of a bonus. Perhaps I'm too traditional but Christmas is the season of giving(presents).

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I plan to give my helper about $20. I give her $20 for all festivities that we celebrate as a family and $50 for her own celebration - which we don't celebrate (hari raya)

I think $50 is more than enough. Alternatively you can purchase a gift instead. :)

thanks