Agree that babies, and breastfeeding, is a totally different ballgame at 7/8 months to how it is at 3 months. I'm still demand-feeding my 3 year old, and please let me tell you, if he was still feeding like a 3 month old, I would have gone crazy a long time ago! In fact he only ever wants a short feed at bedtime and sometimes first thing in the morning, too.
It's quite common for mums/MILs etc to be surprised at the frequency of feeding, firstly because there wasn't as much information about breastfeeding known at the time they were doing it, and secondly, because you honestly forget, in a matter of months, what it is like to breastfeed a newborn. Add a decade or three onto those few months, and they come out with all sorts of rubbish that they insist is fact. Ex-MIL told me that XP slept 12 hours every night from the day he was born, and that he never ever cried. Yeah right! I think if my baby did that I'd see a doctor! Also, your mum probably breastfed for over a year, but also would have introduced solids at 3-4 months, and given the occasional bottle of formula as well, all of which would have contributed to gaps between feeds.
When you start work, your baby will have been eating solid food for a month or two, will be able to drink from a cup if he still refuses a bottle, his tummy will be bigger and he will be more efficient at feeding so will be able to go for longer periods between feeds. In fact this should start happening over the next few weeks, though is often disguised by the 4 month growth spurt. (BTW, if you did want to try him with a bottle again, try the brown latex teats - breastfed babies seem to prefer them.)
Some babies go to nursery, just drink water or juice during the day, and keep their dairy intake up with cheese, yoghurts etc, and then make up for the milk they haven't had in the evenings and at weekends. If you want to return to work and carry on breastfeeding, this is absolutely possible. At 7 months your supply is more robust and can cope with the long gap between feeds, though you might have to express at first to ease the transition.
3 months is so tiny, he will not be as all-consuming for long. Feed gaps will stretch out, feeds themselves will get shorter, your supply will adjust perfectly so that you no longer get engorged between feeds (and most women stop leaking). You won't burn out through frequent feeds, on the contrary, think of it as an excellent excuse to sit down and eat cake!
I also found that once DS was starting to eat solids, we fell into a rough routine and his naps were more predictable as well. I didn't really time his feeds, but they would have fitted into this pattern somehow. At the moment, I wouldn't stress too much about trying to space out feeds as you have the 4 month growth spurt coming up, but instead start looking forwards to the different methods of weaning and decide what route you are going to take when the time comes.