If you don't use deduplication: 8TB for weekly full + (100gb * 6 = 600gb) daily incrementals = 8.6TB per week. 45 days being about 7 weeks so 8.6TB * 7 = 60 TB or so.
If you use deduplication: 8TB for the first full + (100gb * 45 = 4.5TB) daily incrementals (your next fulls should dedup to about the size of an incr) = 12.5 TB.
In both cases, you'd want to have a bit more room for normal growth.
Keep in mind those numbers are very roughly approximated and can vary depending on the type of data you backup and many other factors.
If using dedup, I'd probably try to buy a 20TB disk array (something like 12*2TB 7.2k rpm disks array). Then get monthly stats to see your growth and plan expension from there.
Hope this helps.