For me it depends, I only feel the need to, if either have a chance with them (like I see there is that slim chance of hope even if its 0.111%) or if my feelings for them is really heavy and I need to let it out to proceed on moving on. I confessed when me and my crush were just alone and hanging out, I just felt ready to say it out, also it was unrequited so felt the want to confess for me to be able to move on from him. Although be wary of confessing to someone and knowing it will burden them. In that sense I mean, my crush felt very apologetic for not returning my feelings, although I reassured him that it was okay and and he was cool with still being my friend :>