I did both my PPL and later my CPL whilst working full time. That I work in aviation helped, as I had bosses sympathetic to my desire for flexibility to get the flying in.
Theoretical Knowledge just needs the same study skills you used at school or university - with added self discipline. Schedule in the time you need - say 2 evenings a week and a day each weekend, have a place to do it, keep going through without compromise. It's very easy to let that time get eroded away.
When you get onto the flying, if you're able, some negotiation with the boss about being able to be flexible about your working hours, take time off at short notice when the weather's suitable, and so-on can pay dividends. How easy that will be, will depend a lot upon your job of course.
I found that what worked particularly well during both licences was to fly in the morning, then go to work in the afternoon. I seemed to learn flying much better when I'd not long slept - on the other hand, going to the airfield after a morning at work usually meant that my head was too full of non-flying stuff, and this slowed the learning a lot.
It will take 50-100% longer than you think, just because real life keeps getting in the way. On the other hand, you won't be broke.
G