Talking about citrus that gets overwintered indoors and never goes dormant. Thank you!
I’m sure you’ll get answers from people more experienced than I (for the record I’ve grafted no citrus, but plenty of other plants).
My answer would be when new growth commences in the spring.
I’ve bud grafted citrus indoors successfully which stays in the 60s/70s in the winter. I won’t force growth till early spring here in California. Ideal grafting temperature is 70s.
I wait till the rootstock is growing so that the bark slips off the trunk easily. I also graft when day time highs go up into the 80’s-90’s. In the 90’s i’ll shelter the tree mid to late day an make use of the 80’s through the evening up into the mid days.Grafting Kumquats to Incompatible sour orange rootstock (fruit-trees forum at permies)
I don’t have experience grafting citrus indoors/potted but hopefully its not too different. In my short experience, the most important factors for takes are rootstock flushing, freshness of the scions and weather in that order. For outside, I need to contend with protecting the grafts with foil and batting earwigs that want to nest in it at the same time, but that shouldn’t be a problem for you. For grafting, I found methods with more cambium contact like W/T or bark produced better success than cleft.
I T-Bud all my citrus grafts
My greenhouse gets down to around 50°F, so it’s probably not an ideal time to graft, but the stems on these grocery store fruit made me want to try, so I’ll probably give it a try in the morning after soaking the sticks overnight to rehydrate them a bit.
I’m thinking I might try a bucket thing like @poncirusguy to keep them warm after grafting. I’ve got an assortment of small citrus seedlings to try as rootstock.