Advice on handing notice in

So i have been in my job for just over a year now and have made the decision that i want to look for something else. My job role and the people i work with are great but the company itself has so many problems with it and i need more hours which i am never going to get here, so i have some strong valid reasons for wanting out, however my problem is there are only 2 of us at the moment working there whilst the manager is on leave (she's back in January) which origionally i had decided to start looking then but who's to say she will come back, so if i did leave before then i would be leaving one person on her own which is making me feel incredibly guilty, leaving someone in the lurch like that. Like i said before i do like both girls but the hard truth is they arent interested in progressing in a career and can be a little lazy when it comes to hard work so even though they know how bad the company is i dont think they would leave and the one girl who is still there wouldnt be happy having to do extra hours and more work! I know these are silly reasons to worry about but kind of wanted to get an outsiders view and to see if anyone else has been in the same position.

They're not silly at all! They're thoughtful and considerate. I'd have thought the thing to do would be to explain to any interviewers/prospective employer that you'd ideally like to be able to give your existing employer and colleagues more than just the one month's notice so as to give them time to hire your replacement and have you and your colleague train him or her up before you left? I imagine any new employer would be rather impressed that rare level of conscientiousness, loyalty and job-responsibility...so that'd stand in your candidacy favour, to-boot - win/win. What do you think?