As a general rule, I like to practice several times each day, and I try to structure and plan each one rigorously. I don't always have a lot of time, so I want to get as much as possible out of the time I'm spending with my instrument.
I like to start my day with about a half hour or so of technical work: scales, arpeggios, etc. I like to do this when I'm fresh because, frankly, my brain isn't quite ready for music right after I wake up, so I like to focus on my tone, or something specific my left hand is doing, etc. After that, I feel like starting my day.
My other sessions throughout the day are usually (when possible) about an hour and a half. I don't really like to do a chunk too much longer than that because after a period of time, when I do, I'm not as focused and not accomplishing what I could. Before starting, I map out what I want to accomplish, and as a parallel, when I'm finished I write down exactly what I did accomplish so I know where to start next time (e.g. E major scale in thirds with an a m fingering I got to 73bmp... I'll start next time around 60 and work up from there to a new goal). After I've mapped out what I want to accomplish I start my warm-up with technical exercises and excerpts, all tailored toward reaching those end goals in my repertoire or my playing. This warmup can range anywhere from 5 minutes to half an hour depending on how much time I have. That leaves the duration of the practice to work on repertoire or studies. *generally, it's a ratio of about 15-25 minutes warmup and 60-75 minutes practice. Sometimes that's less if I only have half an hour to practice at this time. I make sure I'm focused and attentive; if I've got something else on my mind, I'll take care of that and practice later; I don't need anything interrupting that focus. I like to do a 5 minute or so cool down after where I'm just 'noodling'...this is more for some unstructured fun, and sometimes reveals new ideas.
I try to get at least two of these sessions in per day. If I have a huge chunk of time available, I'll split it into different sessions with something in the middle that is not strenuous on my hands or really taxing on my mind in the same ways. I might read, take the dog for a walk, go for a short walk, get something to eat. You get the picture...get out and do something else. Then I'll go back and start a whole new session. Multiple sessions are far more successful than on very large section. You can accomplish more, be more attentive, and it reinforces material better.
I don't suggest you have to follow my plan exactly, merely take from it the overall idea: (1) keep sessions short and focused; (2) plan to practice multiple times per day; (3) go in with a plan each time; (4) have some fun! Your playing music for Pete's sake!