Getting a mini implant on upper 1st premolar - will it last as long as a normal implant?

Implants for posterior teeth should have enough strength to resist against heavy chewing forces. In that circumstance, if the material is being chosen as a narrower one than conventional implants, the implant should be a stronger material. Roxolid implants are made for that. If your implant is roxolid, it has an increased strength because it's mixture of zirconium and titanium.

Every clinical situation is different. If your dentist has determined that a mini implant might be needed in the location of the upper 1st premolar, then this might very well be the case. If you do not have the room available for a normal implant then your options might be limited in many cases without more invasive interventions including additional grafting, orthodontic realignment of teeth, etc. Mini implants can last just as long as normal implants if they are done right and utilized in the right situation.

If your dentist knows your case well he/she would have recommended the solution that is most viable for your needs. If you are unsure, you should get a second opinion before placing that implant. Instead, you might have considered having bone grafted in the area that is thinned. This typically rebuilds bone height to prepare for a regular-sized implant. Since mini implants are a lot smaller, in my view, there is a limit to the force they could withstand which could result in a shorter lifespan.