When we did the Plymouth Banjul Rally I think it took us 20 days from Plymouth to Banjul at a reasonable pace with a couple of rest days. You could easily reduce that by one or two days but not much more if you want to have fun and not be rushed. It is too great an adventure to just race down there flat out every day and completely the wrong atitude for Africa. For example, at some police check points which could take a matter of seconds, they might "detain" you for 20 minutes as they insist you take tea with them. Much better to switch to African time and just chill out and go with the flow.
Allow 3 weeks from Rassay to Gambia, another couple of days to get up river to Calum's road and then another few days to get bikes back to Dakar for return shipping and you should be able to complete the journey in 4 weeks.
I'm coming to Skye end august meet and will bring a suggested route and approximate estimate of where we should aim to be each day. NB this is not to over organise it, it is just a suggestion so that we know on route if we are roughly on schedule and can have rest days when needed.