What to do with seedlings

Hi all. First time grower here. Ive been doing some research and cant seem to find any info on this.
So far ive picked up its better to germinate and pot seeds indoors as it has a higher success rate than just planting directly into soil outdoors.
But i cant find any info on how long to keep them indoors? Should i keep the seedlings indoors for a week or two before planting outdoors to allow them to gain a bit of size? or should i plant outdoors as soon as they break the soil to allow them to start of in the proper environment?

If it helps, im doing an outdoor grow in Ireland. Strain im thinking of growing is outdoor grapefruit. I should have the seeds within the next 2 weeks and should be able to start the grow by the beginning of may.
Cheers lads. Apologies if this is a FAQ. I couldnt find the anwser

well, damn, i'd think it'd be a bit chilly in ireland atm... i know around here, mid atlantic coast of the us, the last frost is in may or so, so i'd wait till the last frost is over before planting them outdoors..
i have no idea of ireland's weather cycles, but i'd think they would be some what similar to what we get in the north east.. check out the farmer's almanac for your area for the last frost warnings, then plant them outdoors after that time period..

also, the bigger you allow them to get indoors before moving them out, the stronger they'll be and better off they'll make the transition imo..

If you plan to start them inside, make sure you harden them for a few days before planting them outside.