The airbox lid takes me 10 minutes but I've done a few ...
Allow an hour and that should be fine.
The intake stubs will take you a couple of hours, I suspect. You should also be aware that the socket heads holding the stubs to the head can be corroded in place. It is essential that you use a quality socket head driver and not a basic Allen key to remove them. You can do most of the porting work on the stubs without needing to compare them with the intakes on the head, so you could have those pretty much done and just finish them off when you have access to the head.
Although in theory you should be able to do all the mods within 6 or 7 hours as a first-timer, it's not ideal to put yourself under time pressure to complete everything in one day. Better to have a day in hand to cope with the unexpected, particularly as these bikes are well over a decade old now.