Venus is closer to the Sun than Earth is, and so the solar wind is stronger in the orbit of Venus, an dhas astronger effect on the atmosphere, than it does at the orbit of Earth. Venus also has a slightly escape velocity to retain an atmosphere than Earth.
But at the present time, after the solar wind has been knocking particles out of the atmosphere of Venus for abut 4,600,000,000 years, Venus still has a dense atmosphere, in fact about 90 times as dense as that of Earth if i remember correctly.
And the reason for that is one, Venus has obviously produced a much denser atmosphere than Earth produced, and two, that Venus's escape velocity is high enough that it loses atoms from its upper atmosphere very slowly, just like Earth.
So you can have a planet that is not protected by a magnetosphere retain an Earth like atmosphere for billions of years until intelligent life evolves on it.
Give the planet a high enough escape velocity that air particles have an average speed which is less than 0.20 to 0.16666 the planet's escape velocity.
See Dole, Habitable Planets for Man, 1964, page 35, page 38, pages 53 to 58.
And figure out how close your planet is to its star, and what type of star that is, and thus how strong the stellar wind from your star is at the distance of the planet, and how rapidly or slowly it knocks atmosphere away from the planet.
And if the solar wind will have knocked away a specific amount A of atmosphere in the billions of years it took for intellligent life to develop, and if the intelligent life breaths an atmopshere of density B at the time of your story, then obviously the planet had to start out with an atmosphere density of A plus B, billions of years earlier.
Or possibly the planet will have - over billions of years - been producing new atmosphere at an average rate lower than, identical to, or greater than the rate which the solar wind knocks away atmospheric particles.
This seems to be the same planet asked about in this question:
Climate on a slow spinning planet
And my answer said that I don't know exactly what you are asking for in that question. That makes it very hard to figure out how far your planet should orbit from a star of a specific type. And figuring that out would seem to be necessary for figuring out the strength of the stellar wind and how slowly it would make your planet lose atmosphere.