You want expert for what? There is a huge diversity between STM32 micros. It is very hard to find a contractor if you can't specify relevant info.
Is it a SW or HW development? What HW is involved? Multimedia application with graphics, cameras, audio, wireless? Lora? Wifi? BLE? Battery powered? Application with USB, ETH,..?
Even though I have worked with STM32 for years, even in ST directly, I would NOT consider myself an expert on all of above. If you are looking for a contractor, you need to specify more info, other than "STM32".
PS: It is up to the contractor to estimate the time it'll take, not you.