When did Gero create his androids?

[TheUltimateNinja]

We know it's after the Saiyan arc and before the Androids arc, so somewhere between Ages 762 and 767, but anything more specific?

[Gog]

We don't have any proper estimates, but I'd say around the hunt for the dragon balls arc or earlier? Maybe, possibly?

[Kanassa]

I'd estimate between the end of the Saiyan Saga and the end of Namek

[nickzambuto]

He's been building androids since before the start of the series, as evident by 8's existence. We know he was monitoring Son Goku constantly since the fall of the army till the end of the saiyans battle, but that doesn't mean he wasn't working on them simultaneously. He got all the way up to 18 after all.

Either way, as soon as he was satisfied he made his move. That means he was still working on them during and after Namek. Unless you believe Gero turned himself into an android and perfected 19, then just sat around for three years doing nothing.