The construction of the space will take from as short as 30 days to as long as 60 to 90 days depending on the level of detail, special construction and working conditions. A construction permit usually takes 7 to 30 days for the building inspectors and fire marshals to okay. Preparations of the actual final construction drawings can be rushed for small jobs in 14 days but ordinarily take 30.
As important to the timing of the construction is the management of the critical path, the detailed management of the plan laid out by the general contractor in order to deliver the completed space on time. Every general contractor works off of some form of critical path and shares this with all the sub-contractors and landlord in order that all players in the multi-disciplined construction schedule over many weeks or months are coordinated. Simply ask the general contractor for a copy of the critical path. If they do not have one, then run, or force them to create one. In this way, you will have a written schedule to understand work flow and progress to a timely delivery (and more importantly, an indicator when things are behind schedule, when the contractor rarely desires to share with others).
While construction as an industry is wrought with vagaries and difficulties, acts of god, bad weather, union strikes and simple product delays, construction schedules typically are met, and delivery is timely. But it might not look like that or feel like that when you have an existing landlord, already miffed about losing you as a tenant, breathing down your neck for you to get out of your existing space. It is very common for jilted landlords to behave much like jilted lovers, who hover obnoxiously and make continued contacts after you have already "moved on". Occasionally, with the bad landlords, they will even intimidate your employees by placing phone calls about their needing the space for another new tenant. This may or may not be true, but the legal process of holding over at a premises is a legal process. It is not to be used by your landlord as open season of mental warfare. Should a landlord become difficult or interfere with business operations or employees, ask your attorney to receive the landlord's calls and notify the landlord to cease communications with you.
Only in the extremely rare situations does a landlord actually have a viable ready, willing and able tenant to move in directly behind a departing tenant. And when they do, it usually takes time for their lease execution and construction permits, etc. and rarely is it likely that a new tenant would be lost because of your holdover while ordinary construction occurs. If it persists, turn it over to the attorney. If a holdover penalty is required to be paid as liquidated damages to the landlord, it usually is no more than twice the monthly rent. In the worst case, construction may cause a two to three month holdover and this usually can be solved by money with the landlord, and not litigation no matter how much they gripe.
Nothing contained herein is to be considered legal advice. Always seek legal advice when evaluating any legal document