"the capacity of B. subtilis for induced sporulation reaches a peak about 15 min after chromosome replication has begun. This capacity then declines rapidly, but can be restored by initiating a new round of deoxyribonucleic acid replication." \ref{130463}
"The main stimulus for sporulation is starvation. It is also important that the population density is high." \cite{Errington_2003}

ACTUAL STUFF

Recent reviews of how sporulation is triggered are in \cite{Sonenshein_2000} and this is the paper to end all other research. At least unless there's a newer review, which I haven't found. Until this reaches \cite{Hoch1993}, everything underneath this point will be from this paper.
There are 2 main triggers for sporulation. 
Firstly, population density in the culture. 
"as the mass of a cell culture increases, certain secreted peptides accumulate in the extracellular medium[NOAH NOTE: THIS IS QUORUM SENSING]. When these peptides reach a critical concentration reflective of a particular population density, they are sensed by cell surface receptors"
these then
"initiate a sequential transfer of phosphate groups from ATP through histidine kinases (e.g. KinB, KinC) and two intermediate proteins, Spo0F and Spo0B, to a transcription factor, Spo0A"
The phosphorylated form of Spo0A (Spo0A-P)'s main role during the exponential cell stage is to inhibit abrB, a gene which encodes an unstable repressor protein abrB, which represses many genes essential for both sporulation and the stationary cell phase. NOTE: Re-activating these genes, does not, in and of itself begin sporulation.
Secondly, external conditions (nutrient deficiency)
"sources of carbon, nitrogen and phosphorus can be the relevant limiting growth substrates"
The cells use guanine nucleotides, particularly GDP and GTP, as intracellular indicators of nutrient availability. Internal levels of GDP and GTP are measured with a CodY protein.
"CodY, a global repressor of early stationary phase and sporulation genes ... , binds GTP directly and that the GTP-bound state of CodY is the active state for repression"
Such that "These results lend themselves to a simple model: during rapid exponential growth, the intracellular concentration of GTP is high (1–3 mM) and stationary phase and early sporulation genes (including spo0A) are repressed. As cells experience nutrient limitation, the GTP level drops (because of reduced synthesis of GTP or conversion of GTP to ppGpp via the stringent response or both) below a certain threshold level and CodY loses its ability to repress its target genes"
It's important to note that: 
"most of the genes that are regulated by AbrB and CodY and are induced during the transition from rapid exponential growth to stationary phase are not involved in sporulation per se. They encode proteins whose activities help cells to adapt to poor nutritional conditions by swimming to a new site, by degrading macromolecules, by importing and catabolizing potential nutrients and by killing competitor organisms"
The listed events above are then the cells last ditch attempt to STOP sporulation, by trying other avenues. 
Stage 2 only occurs when
"enough Spo0A~P <accumulates> to serve as a positive regulator of transcription of critical, sporulation-essential genes (e.g. spoIIA, spoIIG, spoIIE) that initiate the σ factor cascade ... "