See also:
- Do English-speakers find it hard to pronounce ‘sixth’?
- How do I pronounce /θ/ after /s/ in English? E.g. in “sixth” /sɪksθ/
In casual speech, I say [sɪkθː], with Compensatory lengthening. Deborah Sale-Butler’s answer mentions “sisths” or “sixs”; I’ll bet you the [s] at the end of “sixs” is lengthened too. In fact, I’ll bet you that’s why she wrote “sixs” and not “six”.