How long after increasing from 10mg of celexa (citalopram) to 20mg will it take to take effect?

7-10 days. Hopefully upping the dose of this medication will allow you to see effects withing the first 7-10 days. It could take 2 weeks but usually a response by that time. Different for each person however. Follow closely with yor mental health provider/pcp.
Around 3-5 weeks. Celexa (citalopram) builds up gradually in your system. You should begin noticing a difference in about 4 weeks, but it may not be fully effective till 5-6 weeks from the change.