Usually 6-8 weeks
Reflexive smiling can be seen from 2 weeks on, but first real smiles (in response to your positive verbal or facial expressions) can be expected at 6-8 weeks.
There is some variability for this milestone, but baby's pleasurable, engaging smiles should become easily recognizable by 3 months. It gets even better, as baby's ability to lough out loud (literally lol) appears around 4 months. Enjoy!